Anything except Sora should do - depends what you have on at the moment. Within a brand, groupsets are mechanically very similar or identical and save weight by specifiying different materials as you go up the range. Upgrading groupset parts might give you a small reduction in weight, but you probably aren't going to notice a change in performance. I'd only go this route if you want to change a triple to a double/compact, or want to increase the number of sprockets in your cassette.
Decent wheels and tyres on the other hand can more than make up for the several hundred grams you didn't save by not upgrading the groupset. Light or aero wheels will be faster accelerating and easier to climb with - they completely transform a ride. £400 could get you some Fulcrum Racing 3s/Campag Zondas/Mavic Ksyrium Elites etc.
The poster's reaction in
this thread from this morning pretty much shows why most of your budget should go on wheels and tyres.