What do I get for the extra £2000

In terms of performance and numbers, in all honesty, there's not a lot in it at all.

https://www.youtube.com/watch?v=3HmVQCAjBE0

I find my Canyon Ultimate is only slightly faster (0.5mph) than my Whyte Wessex over a typical 2 hour ride (including hills) - and that's when I'm trying some, averaging just over 20 mph.

I find the main difference comes in how the bike feels, my Canyon with deep section wheels, aero integrated bar and stem plus a few less grams just feels slightly more special and racey.

I assume being a BMC that it's in Evans rather than online, so go and ride it and see for yourself, then let us know.

Personally, I'd spend the £3.5k on a more special frame and possibly finishing kit to hang the Dolomite's groupset and upgraded wheels off.  For example, your £3.5k will buy you a C60 disc frame, or how about a trip to your current bike's namesake and a Cervelo S3d or Mason frame?

A much higher insurance bill and more paranoia when locking it up, but shiny, shiny.

If you can afford it, spend it on the thing you love.

You get 2 grands extra worth of pride.

I'm sorry, but that's entirely the wrong question.

You should be asking "what else would I spend £2000 on and would it be as much fun?".

That totally depends on what you decide riding a nicer bike is worth to you. 

It'll be a load lighter that's for sure, the contact points will also feel nicer (specifically the new ultegra shifters) and you'll go faster because you have a new bike. 

Worth every penny I'd say