The best you can afford.
I don't shop UK a lot, but this seems like a good deal for that range:
Gigabyte 7950 3GB AMD Radeon Graphics Card
£249.70 & this item Delivered FREE in the UK with Super Saver Delivery.http://www.amazon.co.uk/Gigabyte-7950-Radeon-Graphics-Card/dp/B0071LP5ME/ref=sr_1_2?s=computers&ie=UTF8&qid=1352496523&sr=1-2
If you can find a GTX 670 in your price range that would be better. A GTX 660 would be a little bit less powerful than this if you find one on sale.
Tom's hierarchy article should be helpful if you need context: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html