The 6850 would be a good choice and looking at the performance difference on Anandtech bench it performs ~50% better than the 5770. It would still be a long way off the 570 performance wise and if you can find a 570 for under/a little over £100, then that would be pretty awesome. You might have to wait a while before you see used 570 prices dropping below the £100 mark, so it might be better to go with a 6850. It's up to you how long you are willing to wait, but if you are in a hurry then the 6850 looks amazing at that price!
Great card: Asus AMD Radeon OC Direct CU 6850 i run 2 of these bad boys and one only costs around £100 and performs great on battlefield 3 at high settings. All in all 6850 is the best value card at the moment period!