Found on the web reasons why BA use certain registrations for their planes:
G-EU** for the European Union (A320)
G-CIV* = 747-400 with IV being Roman numerals for 4
G-BYG* = Big aircraft (747-400)
G-RAES = Royal Aeronautical Engineering Society (777-200)
G-STB* = STretched Boeing (77W)
G-BUS* = (A320): Play on Airbus
G-LCY* = (E170): London City Airport
G-BNLA = was the first 747-436 in BA service, the reg being picked for Brand New Large Aircraft.
G-BNWA = was the first 767-336ER in BA service, the reg being for Brand New Widebody Aircraft
G-ZZZ* = (First 777s) 'Z' looks like '7'.
G-VII* = (Later 777s) VII is of course the Latin for 7.
G-YMM* = (Even later 777s) These a/c arrived in the Year 2000 (MM)
G-BOA* = BOAC ordered Concordes
G-BRY* = Dash8s origonally ordered by Plymouth's Brymon Airways
G-EMB/ERJ*= Embraer ERJs
G-MAN*= Manchester based ATPs....