Although its a nice bike if you have a search around BSA tend not to make big money. A Golden Fifty frame sold on LFGSS for £45, A frame on Ebay for £35 and a compete Golden Fifty on Ebay for £45 although there's no pics available on the Ebay bike and the listing didn't have the derailleur spec
Unfortunately, its one of those bikes that will have more value in parts, although personally I think they should always be kept complete. The irony these days is that someone could buy the frame and then spend a fortune tracking down the right period parts.
It looks all original except for the stem and possibly the handlebars so as a project I'd say £60-£80