Well, if you upgraded your CPU, you'd need a new motherboard and RAM as well. So I doubt you'd fit in your budget.
For now, I'd suggest you buy a GTX 570. It should be around 260 british pounds, and that's a balance between high-end and budget. It would run most of the current games totally maxed out, including Battlefield 3 (though, shouldn't expect more than 40 fps with max graphics and AA).
When you get more money, you can upgrade your CPU, motherboard and RAM. I guess it would cost you around 300 british pounds, if you want to get decent parts that will last you as long as your Core2 quad and 8800 Ultra lasted.