I saw this yesterday and thought of you Key specs are: AMD Bulldozer FX-6100 3.30GHz (3.90GHz Turbo) Processor • AMD Radeon HD 6850 1024MB GDDR5 DirectX 11 Graphics • 4GB Corsair DDR3 1600MHz Dual Channel Memory Kit • 500GB 3.5" SATA III Hard Drive • MSI AMD 970A Socket AM3+ DDR3 PCI-Express ATX Motherboard • 450W Coolermaster GX 80PLUS Bronze Power Supply • BitFenix Shinobi Black Midi Tower Chassis • 22x DVD±RW SATA Optical ReWriter Drive The 550ti is a tiny bit faster than a 450 - which gets absolutely smoked by a 6850. The i3 2100 is probably faster in most games than a 6100 but we all know that the GPU is far more important. In my eyes that's unbeatable for a prebuilt at £500, would leave you £100 for a monitor.
OP's friend is going to use Scan Finance (unfortunately) to pay for the parts, so he will most likely not approve of buying from anywhere else.