I'm pretty sure it's a BSA, Ther'es a few similar models but possibly a Model D or a lincoln jefferys? they're quite collectable and appears to be in good order for it's age.
It should be ok for ratting at close quarters but you'd be better off getting something a little shorter and more modern.
The collectors will be all over this post soon with some deffinate answers, I'm just begining to start my collecting career and know not a lot about these things?
ATB, PAUL ps nice find