Sorry, but you really won't be able to do much with that little of a budget. Most high end games (especially played in high resolutions) need at least 2GB+ of GDDR5, that's for low settings and low FPS. If you want playable FPS with higher settings, you need at least 4GB of GDDR3 or (recommended) GDDR5.
I would save up another £100 or more if you want a serious GPU.