Purely rumor and speculation.... and my bad memory...
I discussed the pro's and con's of buying a G-reg vs N-reg in London a few years ago with a local FBO, and the "answer" to how an N-reg aircraft could be used for commercial work was that the aircraft was hired out sans pilot, and the entity using the aircraft had to provide the pilot.
Throw in a special purpose company to be this middle man and bob's your N-reg flying uncle.
Sounded like to much hassle so didn't bother validating against the regs.
This was in ~ 1999.
Can anyone here comment if this was actually ever (or still is) the case?