Looks like a later model, I believe 1920’s -1950’s? Octagonal barrel is supposed to mean pre-war.
Value depends on the condition, eg:the Nickel plate on the cylinder etc, and wether it has ‘Original Will’ markings (Oscar Will) or ‘Original’ or no markings at all which mean it’s a copy.
Maybe £300-500
Video on them from AirArmoury:
https://youtu.be/x80G4ginncw
Cheers,
Matt