I can if you're looking at buying new, but if you're after a used/refurbished machine like that one, eBay probably is your best bet, or maybe Play or Amazon or something. To be honest, you'll have a hard time getting a decent gaming machine together for £250, new or otherwise. I'd recommend buying the components and getting a friend with technical knowledge to assemble the hardware so that you're paying purely for the components and not labour/shipping/warranty costs.
For £250, your best bet is probably an AMD processor with integrated graphics combined with 4GB RAM and a 500GB hard disk. Adding to that the other components, you could probably do it within £250. If you'd like to buy one pre-assembled, check out Novatech, Aria, Cougar Extreme, Ginger6, Power Computing, Computer Planet, PC Option, Element Tech or Arbico. When I was looking for competitive quotes for friend on a pre-built system, those companies looked best for not adding too much extra on top of the parts cost.
If you have a friend who could assemble the components, first post in the new builds section of the forum to get advice on the best options for your budget (people will be able to give more specific advice there) and then look at Aria, Novatech, Scan, eBuyer and Dabs to find the best prices on those components.