Why for example would BA order GEs and then RR engines? Are there efficiency and range differences?
If I remember right, they weren't satisfied with the performance of the GE90s, so went with Rolls-Royce for their next order.
The GE is better at longer ranges than the RR, what drove the GE decision at BA was a combination of factors:
1. Initially, GE threw in a MX deal that was too good to refuse, so BA bought GE on their first 777s
2. The early GE90s had teething problems that BA was not happy with
3. BA had deposits on several more 747-400s with RR engines, then wanted to downsize
4. BA used the deposits for 777 with RR
The regos indicate which ones are in which bucket, as BA has a system of inside jokes for these:
G-VII* for the early GE 777s (VII is Roman 7)
G-YMM* for the later RR 777s (YMM = Year 2k, roughly when they were ordered/delivered)
There are also a few oddballs (e.g. G-RAES is a GE, named in honor of the Royal Aeronautical Society).