So how does the pilot navigate cross-channel? I would say that it is not unreasonable for a trip where the aircraft might be out of sight of recognisable navigation features for a school to ask for a checkout, considering the navigation test in the PPL skills test only allows for use of ground features. Where is it written that trips to the IOM don't need a check? Of course trips to the IOW don't go out of glide range, let alone out of sight of land.
If it is done by an instuctor, and the instructor logs it as P1, instructional time then the other pilot must log it as dual instruction, as my log book would have it, I assume that is P/UT to others. You cannot log P1S in a single-crew a/c unless it is a flight test!