Which Groupset?

105 5700 is barely £100 cheaper than 6800, I'd go Ultegra all the way. Though if you were currently running 8/9/10-speed Shimano wheels you'd be looking at a wheel (or at least hub) upgrade to take 11sp.

Pretty sure you can get 105 for more like £350 if you hunt around and it's still a saving. The main thing is having 10 speed rather than 11, to avoid any hassle with wheels or ongoing chain and cassette premiums.

But yes, if I was investing in a complete new groupset I'd do the same. Although Ed's mix 'n match idea isn't bad. I can't imagine the f/r derailleur would make any difference.