but someone in BA thought that G-ZZZ_ and G-VII_ were good for their 777s...... And to put G-RAES on a 777 for the RAeS I'd have thought quite a high-level decision.
Yeah.... and in the opinion of some (self included) not the best one ever made...RAES in particular gets a lot of stick from one or two in high office because they have to deal with the paperwork and certificates associated with the airframes.
I know some of the public and marketing might like the fancy letters but the trouble for the end users (pilots, engineers) is that out sequence registrations can make the deciphering of the fleet/type manuals and even paper checklists (when used) less intuitive than perhaps it should be. In the case of the fleet in question you've got one sub type (if you include the A market frames') with registrations that use three different lead letters (i.e. R---,V---, and Z---), scattered about the alphabet. Fortunately at BA at least the other two subtypes ( the RR-200s and the -300s, stayed coherent as Y--- and S---).
Oh, no idea who actually picks the letter - probably either the boss or a committee somewhere!