are corssbows even legal??? i'm not sure about that, as for air pistols i'd say go over to Callan's in liverpool.
or there's a gunmaker in chester behind tesco, forgot the name of the shop, but there's there too. they might sell crossbows as well actually. i know they sell shotguns, like proper actual shotguns. loads of them behind the counter, feel like walking into a wild west armoury walking in there!!!