Computers are quite cheap these days, and you should easily pick up a decent system for under £700.
Drop by your local pc world if you have one, they will have plentyof systems on offer, and be able to advise you on what to look for.
Alternatively drop by Tiny.com (specifically this model
http://www.tiny.com/packages.php?prodid=13106 ) for lots of deals.
In general you want to look for the highestprocessor rating you can find (typically 3,000 or sometimes expressed as 3Ghz) and AVOID any pc that mentions "integrated graphics card". These are cheap and nasty, and no good for games. Instead look for computers mentioning "nvidia geforce" or "ATI radion"
Heres a rough outline of what you should be able to get for £700
3 Ghz Processor (AMD or Intel, AMD will display as 3000+, intel as 3Ghz)
1GB Memory ( dont go any less than 512mb)
200gb HD (anything over about 80 should be fine)
Geforce 5200 or ATI Radeon 9200 (any numbers higher than that are better versions of these cards, dont go any lower and DONT get "integrated")
Happy shopping!