Originally Posted by
Mil-26Man
FGR1 I'd imagine
I've never seen any official source refer to the F-35B in RAF/FAA service with any "mark number". The RAF website (never a reliable source for official designations anyway) calls it the "F-35B Lightning"; the RN website calls it the "F-35 Lightning". Note the "II" seems to have gone missing over the Pond, but strangely not the "F-35".