Converting pounds -> dollars won't do the trick. Everybody knows that normally hardware's price is about the same number even if they use different currencies.
First tell us which store will you buy the computer from ( would be good if it was in your native currency ), what will you use the computer for, when you'll buy the computer, if you want to overclock the CPU/GPU, if you need OS and peripherals...
If you answer all these things we'll be able to make a better configuration for your needs.
P.S. Don't buy laptops/pre-built desktops - they're waste of money.
It's not just because it reaches 30FPS in BF3 ( which is quite low ) that you should choose that card.