In reply to Wiles:
I'd be inclined to work it through as follows:
1 - Frame - something the right material, shape, stiffness and weight for what you want to do.
2 - Wheels - the single most important component IMHO. Trouble is, they all sort of look the same. It's easy to identify 105/dura-ace/record/Di2 etc - full sets or part sets and say this is better than that. Wheels - not so easy - but every wotsit of effort will go through the wheels, every bit of freewheeling, every corner, and every bit of braking. Spend the money on wheels, and cut back on the fancy clicky bits.
If you look at most bike specs, and then price up the wheels separately, you'll be somewhat dismayed at how cheap the wheels are in comparison to the rest of it, or find that they're own branded stuff of indeterminate pedigree.
IMHO, that's a strong argument for going to someone like Ribble/Planet X and seeing what you can build.
3 - drive train - 105 is solid and durable. Spend your money elsewhere
Alternatively, go somewhere that has a rather different outlook and buying power - the top end Decathlon bikes are excellent (if you're not precious about logos). They have something nice for £1600 and something lovely for £2,000.
Final option - keep the frame. Get some wheels built up with Chris King hubs and Stans rims, put tubeless Schwalbe 1 on them (about £1000, of which the hubs will be c. £750) , buy a big box of Ultegra from Wiggle (£500) and enjoy. If there's any spare cash if you replace saddle (Fabric scoop £40), seat post and pedals, get your frame repainted (Ooey custom paint).
Some time later, buy a nice frame. The hubs will run for years - just replace the rims from time to time