Are you sure about that? 77L is the usual the airline code for the 777LR version that was simultaneously launched with the 77W (300-ER). Boeing based the 77L on the 777-200 length fuselage but with the higher thrust GE115B engines from the 77W.
BA assigns internal config codes to its aircraft and here 77L relates to the 48 Club Suite Seat Aircraft. (Others inlude M, J, G etc.)