IIRC, the ones carrying proof marks were for the Australian market originally, because their laws required anything above .177cal to carry proof marks. The smooth-bores were primarily for the Irish and Indian markets, but were widely used in seaside/fairground shooting galleries. Some of the proof marked ones ended up on our market because BSA either made too many for the export order, or the order was cancelled.
I also seem to remember a batch of the smooth bore ones was stolen sometime in the 1970s (and I don't think they were recovered), could this be one of them?
For the Mk1, I'd say somewhere in the region of £70-£90, the fact that the smooth bore has no sights, and is on the rough side would put it in the £20-£30 range.