£900 is a very generous budget and you can easily build a configuration that's able to play all current games in all their graphical glory
Here's a few UK suppliers:
- SCAN, EBUYER and AMAZON and probably the best of the bunch. Keep an eye out for shipping costs too, just because the product is cheaper on one websites doesn't mean it won't actually work out more expensive with delivery costs.
EDIT: I suggest creating a new thread and use this build advise template ( http://www.tomshardware.co.uk/forum/261222-13-build-advice
) in order to see what components people suggest are best for your budget.