I believe that the ZZZx was chosen because ZZZ looks a bit like 777. YMMx was chosen because the year 2000 was coming up when it was first allocated. Not sure why the BA-ordered 734s got DOCx, but GBTA was to 'honour' the Guild of British Travel Agents, and GBTB just followed on from there. The last few 744s are in the BYGx series, which I assume was just BIG spelt wrong!
The GB Airways aircraft are G-TTOx (320s) - T(hree) T(wo) O(h)and G-TTIx (321s) - T(hree) T(wo) O(ne). Their old 737s were G-OGBx - self explanatory.
Anorak aside - ZZZD and ZZZE are now with Varig as PP-VRC and D respectively.