I'm no expert, but from what I've watched go through on here over the last year or so I'd say - if its a .22 then around £240 - a couple have sold very quickly on here for a little less. The .177's are rarer and so fetch a premium. I got a bargain with mine at £220 the only other one I'm aware of being sold nudged £300.
I'd never really thought of mine as a "collectable" but they seem to get snapped up quick. If you are wanting to sell it there is a chap on the BSA owners forum wanting one.