Can't help with 'MM' but I think 'EU' is on at least one of the GB Airbuses.
Regarding 737s, several of the BA 737-300's had 'regional' regs such as G-XBHX (Birmingham) and G-ODUS (Dusseldorf) which I assume was someone's attempt at highlighting good old 'Regional's routes and bases (RIP
). I've been away for a while and maybe these have all gone? I know BHX is on another reg now. Some of the other 300's began G-LG--
A lot (All?) of the 'BA Spec' 737-400's began G-DO--: Others (presumably ex GB)) were G-GB--
The ex DanAir 737-400's had the DA regs still.
The 737-500s appeared to be totally random as G-GFF-, not that I can make MUCH sense out of many of the others!
I feel sure there is a website somewhere that lists ALL the BA regs, and that if no-one pops it up here you can search Pprune or Google for it?