At what point do you enter or leave the UK?

I hesitate to share another Absurd Tax 'funny' so soon after the last one but it's worth it.

The tax rules for counting days of residence in the UK have changed recently making this question more relevant than ever before. Previously days of arrival and departure were not counted.

The new legislation includes a clear exemption for any day on which someone's presence in the United Kingdom is solely as a passenger in a part of an airport or port not accessible to members of the public unless they are arriving in or departing from the United Kingdom.

HMRC have therefore confirmed that changing planes in the UK (eg: on a flight from the USA to a European destination) would not be counted as a day of residence here - as if anyone could seriously have suggested otherwise.

So in the case of flight delays and trips in and around 5 April, how far do you have to get through the airport to have entered or left the UK? Assume you're leaving, is it enough to have gone through passport control? Customs? Into the departure lounge? On the plane? Flown off?

And of course the reverse would be true for arrivals. I can see it now, someone lands in the UK just before midnight and loiters before going though passport control until after the clock strikes 12.