Hi,
Post-war Webley Juniors in similar condition to the one in the photograph are quite common in the UK and would normally sell for £40-£60. I wouldn't consider it to be in 'collectable' condition, but it might do until a better example comes along.
You are probably already aware that post-war Webley air pistols can be roughly dated from the address stamped on the side, with those carrying the 'B4' post code dating to earlier than 1958 when the company changed premises.
I have this unusual 'B4' Junior with some engraving applied to the breech block by Webely apprentices to disguise minor casting imperfections and allow the pistol to be sold rather than scrapped. These are quite rare with the engraving differing slightly depending on the particular apprentice who carried out the work.

Regards
Brian