The chosen systems, including graphics card card is overkill for your needs. I could suggest a suitable one for quite a bit less.
Also consider buiding it yourself (click here), but if that's not an option & you still want to stick to your budget...
I would go for this Dell XPS 420 click here, remove the monitor, but select the 3Gb RAM upgrade & Logitech MX®3200 upgrades = £494, or £473 with cashback (click here)
Then get a larger 22" monitor with DVI, a 500Gb external hard drive* & better 2.1 speakers: click here, click here & click here = £217 (use free del click here for the price)
Total price = £711, or £690 with cashback. G
* Making it 1TB in total, but only if you require the extra space. You could just save the money & get the above for ~£655, or £634 with cashback
P.S. Read the Logitech MX®3200 user reviews at Amazon click here - some have complained that the letters are too small