BA chose that engine because it already has 742, 744, 757s powered by the same family of engines, but i dont remember who else uses RR 767s.
The lower thrust is not an issue, it has no affect and BA use 767s very sucessfuly transatlantic.
Most airlines have PW or GE because they are only double stage engines. The RB211 is a triple stage engine which makes parts more costly. But apparantly the RB211 is a lot more fuel efficient and has more potential expansion is RR went ahead. It almost did for the 764 but because only a handful of operators use the RB they didnt bother. And to date, only two operators have the 764, DL, CO.
Long live the 767 and RR!