|That's my best guess using values in the US. |
I know pc components are more expensive in the UK (ongoing complaint from the UK guys here) so you might be able to get more.
Advancing technology also has an big effect on prices when I can buy an AM2+ motherboard, processor, 4 Gb PC2-1066 ram, and nice video card for ~$200US.
Even if your rig was nearly new, the fact that it's used goods would cut the value by about half. You can't offer a parts and labor warranty so that cuts the value even more; the 8500GT kills you with the gamers; folks who just want to surf the internet and do office work would probably look for a less expensive machine when buying used.
All the above can thrash the value of a machine in which you've invested £600 or more.
A look around auction sites in the UK should give you an idea of what you can hope to get back from your investment.