The daft rule applies across the board in the UK. Shortly before the covid downturn, I flew Dubrovnik - Birmingham - Manchester with Freebird Europe.
Guess what? We all had to get off at BHX, go out through arrivals, join the lengthy security queue in the departures building (PIA B77W pax in the mix), go through the full security thing AGAIN ... and then get back to the aircraft. They told us we had an hour to do all this. Inevitably, it took much longer. Especially for me, as the gate staff couldn't find the transit boarding pass with my name on as we disembarked the aircraft. I was given a hand-written one about ten minutes after everyone else had set off. The whole thing is an absolute nonsense, of course.
But seriously, if a bad actor was intent on mischief, would they sit there quietly on the three hour DBV - BHX leg, risk getting caught at BHX security (if somehow not detected at DBV), and then do their worst on a 15 minute BHX - MAN hop?
Only a BRITISH DfT official could come up with a system so utterly ludicrous. Knowing what they've done to railways outside the SE as well, I can only despair at the level of talent evident in the DfT.