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.
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.