ID check is to prevent arriving international passengers ostensibly connecting onto another international flight from simply boarding a domestic flight and missing the UK Border check on arrival in say, Glasgow. The whole rationale came about when they stopped segregating domestic travellers as they wanted them to spend in the same shops as transit passengers. Hence when a domestic passenger arrives off the street into LHR, they get their face scanned to make sure they don't simply off-load, leave via the UK border and allow someone with no right to enter the UK to use their boarding card on a domestic route and avoid border controls.
The case of T2 domestic to T5 domestic is laboured, *BUT* the numbers are tiny as you're talking about connecting EI's BHD or FlyBe's ABZ/EDI/IOM/(JER as CTA)/NQY to BA's ABZ/BHD/EDI/GLA/LBA/MAN network, and there are easier ways to get from A to B on that list rather than via LHR.
btw as soon as you got on a bus at Flight Connections you were mixing with other international passengers and so as a non sterile environment, you'd need to re-clear security to Uk DfT standards at T5 unlike at T5 where domestic arrivals on the 500 stands to about 506 can go upstairs directly to departures as they're kept in a sterile area within UK security standards. #phew