More to the point, is there any prospect of an initiative from UK airport operators to get this archaic restrictive throwback reviewed and rescinded? There is no justification for this nonsense to continue. Of course, this would imply Whitehall mandarins actually doing something sensible, and I know that is a very big ask.
Back in the day, MAN had routes such as LUX-MAN-DUB (Luxair) and LIN-MAN-DUB (Alitalia). Such routes are essentially impossible to run now. Double-drops were once a sensible solution for certain carriers; they could be again if dumb bureaucrats stepped out of the way.