Returning to my original post, I think my bemusement was mainly at finding myself on the external side of the UK border, despite never having left the country. If I'd not had my passport with me, but had say travelled on Flybe using a driving licence (as I need photo ID on their flights to satisfy their Ts & Cs) then I presume I would have had to have approached someone at a border desk and explained my situation. I guess they would have been able to check Flybe's passenger list and let me through, though I can't help feel that there may be some potential breach of a right here as I should be able to travel within the UK without let or hindrance, and I'd include in that explaining what I'm doing. I'd quickly add that it is not something I'm actually taking issue with, more theorising.
As a general point, I wonder if Heathrow should put a notice in the domestic transfer area of T2 warning passengers that if they are transferring airside to a domestic flight at T5 they will encounter the UK border? Passengers could then, if they wished, choose to transfer landside, with no such issue.