What resolution do you want to play at? That really makes a difference on what to recommend, as does budget.
Try looking at this article for some ideas (though the prices aren't in pounds): http://www.tomshardware.com/reviews/best-graphics-card,2521.html
The article suggests the GTS 250 isn't significantly worse than the 9800 GTX. I'd suggest sticking with the GTS 250 or going with a Radeon 4850, or if you can afford a little more, step up to a Radeon 5750.