Light bike on a budget.

My Trek Emonda with 105 and Kysrium wheels is under 8kg and cost well under £2k. I had a Hongfu FM066 with a Microshift Arsis groupset and that was nearer to 7kg and around £1k.

It's just attention to detail and not just heading for all the well-known brands.

For just a smidge over £2000 you can get a Rose X-Lite Four with either Ultegra or Force 22. The Force 22 one weighs 6.7kg. The Campy Chorus one is significantly over your budget. 

Alternately the X-Lite Two is well under budget and weighs 6.8kg but there is no Campy option. 

A framset option would be this:

https://www.cyclerepublic.com/boardman-elite-slr-9-8-frameset-race-plat....

£720 with British Cycling discount which would leave you enough to build up the bike within your budget. The frame is 798g so super light and you don't have any potential issues that can arise from buying straight from China such as import taxes. 

Sub 8kg for £2k should be easy.  I'm currently buying parts for my first carbon bike which will be a Cinelli Saetta with Campagnolo Centaur and some handbuilt wheels.  Nothing special in the finishing kit.  It's likely to come in at just over 8Kg, and I'll have spent about £1,700 on it, plus a bit extra for some Campagnolo specific tools.  If I spent the £300 on carbon seatposts and bars and whatnot, and went up to Potenza instead of Centaur, I could easily get it under the 8Kg mark, and that's without even looking at a particularly light frame or wheels to start from.

Second hand.

Ebay or similar. 3 years ago I bought a Colnago C60 with full duraace that had done about 2000 miles. It was in pristine condition and cost $2350 ( GBP1100). That came in at just over 7kg. This week on the same auction site there is a Cannondale Supersix, 3 years old, full DA for $2500. This is a bike that is $12k off the shelf. I race one of those and in full race trim it is 6.55 kg (52cm). Honestly, the value for money is just crazy.

This would leave a good amount out of your budget for more pies.

As for which bike? Truly, once you get into superbike territory (above GBP3k) it simply comes down to personal preference, any performance difference is vanishingly small, or in the head. I've raced for 30 years and simply the best handling, fastest, most comfortable bike I've ridden is my Cannondale supersix evo. I didn't like the Colnago, it was too much of an armchair ride, and felt like a ferrari compared to an ariel atom against my Cannondale. I've ridden a Ridley Noah - very good; a Cervelo S5 - Felt 'clunky' but was damn fast down hill; A Felt which was so close to my 'dale I probably couldn't tell the difference; Argon 5 - a 'lumpy' ride; Scott's similar geometry to the Colnago - didn't lik e it; Specialized Venge - 'soulless' but with Di2 was almost like not riding something.... I'd be happy owning any of them except theScott or  Colnago (sold the frame and ended up with a near new DA 9000 groupset  and wheels for $800). And once, a titanium De Rosa with Superb Pro that I would sell my soul for. 

Hong Fu FM066 frame, some low profile FarSports wheels, whatever Campy groupset and finishing kit you can afford, should be well below 8kg

I think you should spend the money on pies