For UK mainland to IOM, you fax the GAR to Special Branch (police)
only. Customs/Immigration not involved - AIUI.
Just out of curiosity, are foreign registered planes / foreign pilots / foreign-licenced pilots, or any combination thereof, expected to jump through those same hoops?
Yes.
However, a "genuine foreigner" would probably fly to the IOM (or Ireland - same scenario) direct from some country other than the UK, so would be subject to the IOM's own notification requirements, which are a bit more reasonable than those of the UK mainland police.
The "better funded" pilots tend to deal purely via handling agents and the agent takes care of all this crap for you. He needs to know, for everyone aboard, the name address passport #, which I think he needs to know anyway.