I'm in the UK too, I can recommend the following sites, I use them often:
Also, if you DO want to go for the same model of graphics card you can get it here
a few quid cheaper, plus free delivery. Also it doesn't have on of those big ugly heatsinks.
Keep in mind these two things: 1) You will need at least a 450W power supply to use any card like that, according to ATI's specs 2) If you don't have a DVI-capable monitor, you'll need a VGA-DVI converter.