I thought inbound passengers & crew who remained 'airside' were legally 'in transit of UK airspace' until they cleared immigration control.
This has poss implications for foreign sailors (fisherman) in UK territorial waters
IMO The Govt would also have to record and tax foreign lorry drivers income for the time spent in the UK and illegal immigrants held in UK detention or for earnings whilst on the loose (to be applied & deducted from their repatriation incentive
How about time spent overflying UK airspace eg International Space Station
Meanwhile those who can afford creative tax accountants will avoid payment