The BA 767-336s are unique to BA in that the engine is retro-fitable to the B757-236 and the B747-436 in the BA fleet almost off one wing type, and onto another... Fine if you need that flexability, but not so if you just need a B767 to operate low-cost flights for you. Granted RR powered in common with existing jet2 B757 acft, but the B767 with RR engines is not too common.
BA faced this issue when trying to sell some of the 747s to Cathay. Cathay in the end went to SQ and took some of their B747s which had a completely different engine to existing CX B747s.