The lugs are the same design as Lambert used on their frames in the late sixties and early seventies, although there are plenty of other details (including ironically the lack of a wrap over seat stay) that suggest it isn't a Lambert:
http://www.classicrendezvous.com/Britis ... _DBs_1.htm
The deep drop handelbars look a bit Lambert /Viscount as well - do they have a maker's stamp on them?
If you can check the seat post dimeter, it might be possible to take a stab at guessing what the frame's made from.
It's a handsome frame, and 23lbs is nothing to be ashamed of for a fully built up bike.