what kind of spec are you looking at ?
- mostly which cpu and how much ram ?
guessing at £50 you're looking at a core2duo.... you'd be better spending a bit more and getting something with an i5
on a bye it now you're looking at £100 is, but if you keep an eye out on auctions you could get one for not much more than £50
keep a lookout for dell optiplex systems - they'll almost all be ex company machines that have been replaced - they'll have decent components inside from dell (not cheap crap ram etc.)