Re no APD on separate tickets on UK transit - how would you police so that people don't just claim a spurious cancellable inbound booking on a different carrier, Eurostar etc.? To cover this potential evasion, it would have to be an after the event refund from the outbound carrier, rather time consuming to administer. I doubt that the outbound airline would wish to be involved, especially if it wasn't their flight inbound. Even presenting an inbound boarding card would require further research to establish UK scheduled arrival time as being within 24 hrs.

In any event, the 24hr limit ties in neatly with BA's stopover/connection limit on discounted through fares ex Europe and elsewhere, so for many I suspect it's not an issue.