AMD Radeon HD 6850 or AMD Radeon HD 5770

6850 is better by a good margin and the best for its price the 6870 is better and stil under £120 see http://www.aria.co.uk/SuperSpecials/Other+products/MSI+... you may find one cheaper if you shop around.
Check
Ebuyer.com
scan
overclockers
pixmania
novatech
dabs
and any others you find the offers they have vary quickly.

The 6850 will perform noticeably better then a 5770 for gaming. Especially at higher resolutions due to an increased memory bandwidth (6850-256bit, 5770-128bit). It also will draw less power, generate less heat and run more quietly. A 6870 is also a good choice to look into for slightly improved performance and more overclocking headroom.

What is the rest of your system?

These are just numbers, but I personally love this site for getting a rough idea of how cards stack up. http://www.hwcompare.com/5917/radeon-hd-5770-vs-radeon-...

I agree with the others here, 6850 gets my vote as well.

Here is a comparison of these 2 cards in crysis
http://www.youtube.com/watch?v=Bsjabpq4dFE

You can clearly see the major difference...