Originally Posted by
warakorn
In the UK (e.g. LHR) - this is my understanding - the passenger has officially left the UK, once he passes the conformance/security checkpoint. Because if the passenger wants to get back landside, he has to go through the UK border checkpoints.
I guess this is now offtopic as it has been moved to the US forum, but just to say this is not true, because in most (maybe all) UK airports domestic and international departing passengers are mixed. Furthermore, in some airports (such as EDI) I think you can be departing on an international flight but still just walk out of the secure area via domestic arrivals (getting back in may be problematic).
For visa purposes, I believe you are considered to be in the UK if you are physically on UK soil at any time during a calendar day, unless you are in international transit and have not passed through the border - but it is slightly odd that at LHR you can still physically interact with a passenger who is travelling domestically and has no ID on their person.
For tax purposes, you are considered to be in the UK if you are on UK soil at 2400, but there are so many rules and exceptions. For example, if you deliberately scheduled the aforementioned international transit via the UK in order to have a drink with the passenger travelling domestically, inside the airport, you may be considered to be in the UK for tax purposes on that day, depending on your other activities in the UK for the preceding 3 or 4 tax years!
If your flight is scheduled to depart at 0100 then I believe you would still be considered to be in the UK on that day even if it's only for 1 hour. But I would expect discretion to be shown, as in not considered to be overstaying, if you were scheduled to depart at 2300 on the last day of your permission to be in the UK, and were delayed past midnight.