the RB211-535E4 fitted to the B757-200 is a different spec engine to the RB211 variants fitted to the BA747/767 fleet, true the engines are interchangeable between the BA 767s and the 747s, the joke was would you rather cross the atlantic with two new engines or four knakered ones as the low time / refurb engines were fitted to the 767s and the high time engines to the 747s! Some years ago there was a program of updating these engines with RR trent cores but whether any of these were fitted to the 767 i do not know.
The BA 767s are a white elephant, they are unique in the fact that they are RR powered and in the past not even the RAF would buy them from BA. If they were cheap enough i could see PM buying some, BUT i very much doubt it due to the spares problems associated with the BA airframes. It is rumoured that IF PM does buy any 767s he wants GE powered airframes.