For your budget you are limited and will be hard pushed to find a PCP match pistol, I did sell a Walther LP300 for exactly that but it was in all honesty underpriced to help a local shooter get started.

SSP
FAS604, great pistol. Well balanced and excellent trigger, As accurate as anything else out there, they also hold value well.

CO2
Tau 7s are good pistols for the money, if it's used check the barrel for rust, if lubed correctly they are normally ok, they have a good trigger and are superbly accurate,
Early CO2 pistols also worth considering especially Walther CP2, CP1 etc, FWBs now starting to get collector prices

PCP
Morini 162 (fixed cylinder) can occasionally be found for less than £500 but ensure cylinder is ok, also uses old type batteries that can be a pain to get hold of and has a less reliable trigger than later models.
Early Steyr LP1s occasionally come up but rare for less than £500 and you'll need to be quick if one does. Great pistol if you do find one.

ATB
Nige