Originally Posted by
Vortex_Generator
I'm aware of this, I just wondered why some are allocated UK designators whilst others are not.
Ah, your OP seemed to suggest that the first group did not retain their US prefixes.
https://www.raf.mod.uk/aircraft/globemaster-c-17
In UK service, the type is typically referred to as C-17 or Globemaster, no formal RAF role/numerical designation (Globemaster C.Mk 1 would have followed the regular pattern of aircraft titles) being applied, since the aircraft was initially leased.
That doesn't explain the others, though. No idea on those.