A banger is one who bangs. A British banger must, therefore, be a bangin' Brit. There is a wonderful old South-Central-Highlands-Of-The-Lake-District-Middle-English folksong (HA! a music reference!) called "Bang Away Lulu" (or "my Lulu") which makes it clear that Lulu's banging isn't on a drum (shall we say?). Therefore, a British Banger must be a sexually obsessed Englishman. But as we all know, "sexually obsessed" and "Englishman" are contradictary terms (except for certain members of the Royal Family, but they were originally of lusty German stock).