What is it really like owning a high end road bike in British winter?

Depends on the Superbike.  I did about the same mileage as you last year mostly on my Reilly Gradient.   With the possible exception of the frame the rest of the parts are pretty much what you'd find on a mid-high end bike - GRX DI2 and a power meter in my case.

Being a Gravel bike in the winter it gets a fatter tyred wheelset and mudguards but the consumables are either the same or identical spec.  (Same wheel bearings across the wheelsets), same BB submerged in puddles etc etc.

Bear in mind there is marginal difference between the high end consumable parts than the mid range ones so they are all built to be fairly robust.  Headsets, BB's, chains, rings - they are all much of a muchness once you get past the all-steel low end stuff.

1.  No.  2.  Yes.  3.  No.  4.  Some do, some don't.  5.  Depends on the tyres you put on them.

My 'good' bike goes to bed between November-ish and April-ish. For me, it's the fact that winter roads knacker moving parts like nothing else. It's partly the grease and mud, but mostly the grit and salt. Full lenth mudguards help a lot, but they don't solve all the problems.

My customers with the top end bikes only use them on dry days....and they are already in the Hung up until next year or booked in with me for a service and then hung up. Some are indoor warriors.... most are Gravel/Winter or both cyclist until Easterish..... infact many keep the Gravel/Road mix all year... and use the winter bikes on wet summer days....  As for them being "flaky" some are very and need regular fettling....like regular and as we know, race bikes are washed and set up daily after 100+ miles or so...... I've a mid range Italian Campagnolo equipped bike for the summer and it's hung up already....the other bikes I ride are already in action until we have a sunny day....   I've 4 Turbo trainer bikes to sort this week...on site... in man caves and one lady cave

My Canyon Ultimate CF SLX is at the lower end of the high-end road bike category (the only difference is I chose not to go for Di2). I choose not to ride it through the winter months, as I'm fortunate enough to have a dedicated winter bike (Whyte Wessex) with mudguards, a bike that sees a lot of abuse throughout the miserable winter.

For context, I ride about 2000 miles a year on my Canyon (dry summer miles) and 3000 miles a year on my Whyte (largely wet autumn/winter/spring miles). I hope the following answers to your questions may be of some use.

1. No, not really any more looking after than a cheap aluminium bike, just don't prop it up by the frame or throw it on the floor (all bikes, regardless of cost, should be cared for).

2. Sort of. As I ride my Canyon only when it's dry, I give it a quick wipe down after each ride to keep it shiny! My Wessex sees far more abuse, again I wipe it down after every ride (particularly if it's got muddy) and I clean it every couple of weeks throughout the winter (I'd do the same to a cheaper Aluminium bike).

3. Nope. Lots of people say hydraulic disc brakes are a maintenance nightmare. I haven't touched my hyd brakes in about 2 years, they are still sharp. Gear cables need changing as they wear (as per any other bike).

4. Yes. The winter is harsh on bikes (hence I have a winter bike), for example, I use the same type of Dura Ace chains on my Canyon and my Whyte, the poor weather in which I ride my Whyte means the chain wears twice as quickly. All the other drivetrain components wear and corrode quicker in the winter. I could ride my Canyon all winter and simply replace the pars, but I choose not to.

5. No more than any other bike. If you ride a bike over ice, you'll probably fall off, it doesn't matter if it costs £500 or £15000.

To be honest, there isn't much difference between a ~13k bike and a ~7k bike. The main changes will be full electronic gears and a power meter.

The difference between a ~3k and ~7k bike will be aero wheels and a difference in groupset (the more expensive bike may also have a frame light by approx 200 grams).

Looking at it objectively, the difference between a ~3k and ~13k bike will be about 1kg of weight, switching from Shimano 105 mechanical to Dura Ace Di2, a powermeter and some super aero wheels.

I have space inside but have to carry my expensive bike around tight corners, close to white paint on the wall. This leaves me with the option of using my expensive bike only on dry days and my inexpensive bikes outside all year. I am pretty sure, however, that the bikes left outside and used in the winter need much more service and new parts. So it is perfectly possible to have expensive parts on the bikes used in winter, but in my experience this only leaves you with two (in my view not very appealing) options. Either you clean your bike well after each ride in the winter or you replace a lot of expensive parts each year. Most people I know who owns 10k+ bikes and use them in winter will do the cleaning as well. In my case, I try to upgrade my winter bikes with parts that are less susceptible to weather and theft in order to maintain some kind of performance alignment to those who ride expensive bikes all year.

If you have the storage space I'd consider taking the budget you have and splitting it between two bikes: I buy all my bikes secondhand, I've got a beautiful Specialized Roubaix Dura Ace Pro for "best" and a Specialized Tricross for grim winter days, still a lovely ride but one that's cheap enough and robust enough to take on the worst of winter without taking too much damage, without needing too much cossetting and without spoiling my rides by having me wincing every time I hit a pothole or go through a deep puddle.
 

If you can afford it then why the hell not. £13K doesn't even get you a new car these days - drop £600 a month on a new 4X4 and nobody bats an eyelid ! - A bike will most likely give you more enjoyment and of course there are the mental and physical health benefits of cycling as well.

All that said my foray into high end bikes ended in a expensive disaster - The frame creaked from day 1 and eventually cracked.....and cracked again - supplier refused to help - I had to write off an expensive frame (it wasn't £13k expensive  - but it was way more than I needed to spend)

I sourced a replacement frame from Dolan - which has been a revaltion - best bike ive owned. Its got we wondering are these high frames too light ? - I mean fine if you are a pro - your frame cracks they give you a new one - for mere mortals we are at the whim of the supplier - and even if they do help - you are still likely to be without a bike for a while whilsy they test it etc....

So for me - its Boardmans/Dolans/Giants/Merida - I d like to say I let my legs do the talking ......but they don't these days.

Do bikes like these need to be looked after like a fragile prince/princess?

- Often yes.. road debris will be more common in winter = scratches, plus some posh wheel hubs don't like the wet/damp. 

Do they need to be cleaned after every use?

- That's up to you.. but 'best practice' would be to give it a quick wash I'm sure. 

Do they require really elaborate servicing?

- Nope. Same as every other bike.. (apart from if you have posh bearings in things, they can take things to a whole new level)

Do people who own them keep them for spring & summer only?

- I have a upper middle class bike, it gets used 90% in the dry and only on the odd occasion in Winter. 

Are they particularly iffy when the roads are frosty? 

- Its the tyres which touch the road not the other parts, so no more iffy than any other bike to ride. 

I try not to ride much in the winter nowadays but have done my share of inclement weather rides for sure.. I'd say for any bike, Winter can be hard on them, regular cleaning is a must, when brakes were 'rim' then you could wear through wheels quickly if you weren't careful although I'd like to think that's mitigated somewhat by discs now..  the key things I used to do were fit mudguards badly, get frustrated with them and then remove them and stick with an ass saver.. either change tyres for something larger or just lower the pressure a bit in existing tyres for more grip, be extra careful around roundabouts, never underestimate a puddle, I would also generally have to replace my lower headset bearings once a year after a winter (but that's probably more bike model related than something everyone does).. 

Key takeaway here is, don't buy one high end bike, buy two bikes, one high end one and one cheap one for winter. Winter miles = summer smiles. 

Increasingly capable I think. Most bikes in that category will feature some or all of the following:

Fully integrated cabling
Wireless or Di2 shifting
T47 BBs
Hidden mudguard mounts
Space for big tyres

Probably the only thing missing is dyno light routing and mounting. Stick some grip tape on the exposed tops of your swanky aero bars and you're good to go.

The only risk is cost of fixing it if you stack it on field run off, ice, or from sidewinds hitting the aero wheels.

 No different to any other bike they all use basically the same bits i wouldn't expect them to require anymore or less maintainance.  Frankly a 13K bike is laughable but great for marketing and i'm sure if you've got the spare cash it's easy to justify but it won't make you faster than the 18 year old on a 10 year old bike bought on ebay for £500.   Specialized have just launched a 13k gravel bike which sounds even more of a stupid risk over a 13k road bike.  
The marketing machine seems to have moved on from Aero and seems to be hammering on about how light these really expensive bikes are and again that 10 year old bike is probably lighter......

If you race avidly and frequently, a bike of the type you describe may be worth having - although there's no significant difference, other than price, bling and fragility, in similar bikes costing a quarter or a fifth of the price. The less expensive ones, as others mention, may be tougher i' their parts'; and will anyway cost far less to mend if you break or trash a part. Also, bling of the £12,000 bike ilk says, "I am a dafty but this bike is worth stealing".

However, given your description of your riding, you really don't want an out & our race bike at all. Far better to select a bike fit for your purposes, not those of the racer you might have fantasies about .... but aren't in your everyday cycling.

Around 20 years ago I gave up racing and quickly sold the racing bikes, leaving the tourer. I then got a cyclo-cross bike of the all-road kind - fittings for guards, panniers and other stuff allowing it to be easily transformed into various bike types for various kinds of cycling. The comfort, carrying capacity and ability to go anywhere in any weather have been far more useful than somethng that might go 5 minutes faster for the same energy input over 20 miles.

These days I have various bikes but they fall broadly into summer and winter bikes permed with sporty and carrying varieties. I even have racy bikes but they're now configured for a much more upright position and gears I'll use all the time, as well as fat tyres and so forth. Lowest gear 30 front and 36 rear but also a 52 front and a 14 rear highest, with a middle ring for cruisin'. Good for an ole phart who still loves hills.

Other bikes of mine have motors in 'em! I fear no hills, even them double arra ones.

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In short, why waste money on a blinger for a style of cycling you never do? Be practical, man!  Also, avoid being passed by horribly fit sprogs on a cheap Triban sneering at your low speed despite the TdeF bike.  

Whether a bike is so hallowed either because it's worth £13k or it's an amazing vintage restoration, the dilemma is whether to ride it / ride it in grotty weather. 

My opinion is that a £13k bike is a security liability and that you could have up to 4 really nice bikes for that, each suited to different conditions.  Having said that, I also keep my best (Ti) bike for winter riding - it's an incentive to go, I tell myself. Shame it runs Ultegra 6800.

Don't forget that Dura Ace and the like are the plaything of pro teams for whom money and mechanic labour are no object. 

Do bikes like these need to be looked after like a fragile prince/princess?

you'd expect to get light and strong wouldn't you, but that isn't always my experience of high end stuff (see above) 

Do they need to be cleaned after every use?

yes - protect your investment - you'll enjoy a clean bike a lot more  

Do they require really elaborate servicing?

no more than a bike of £4K. If you've running snowflake modern stuff like BB30 or D12 shifting, you're setting yourself up for the associated problems at any level. It's easy to bash a pedal and whether you've paid £50 or £250 won't make much odds. 

Do people who own them keep them for spring & summer only?

yes - its usual to have a winter bike that runs full time mudguards and has chunkier tyres perhaps with a cheaper/ cheaper to run drive train. 

Are they particularly iffy when the roads are frosty? 
no more than anything else.  Again, though you probably won't want to risk a spill e.g. your CF handlebars or super pricey pedals.