In London .......

Commonsense, followed up if necessary (which will only be for a few) by immigration staff, who will come round and see you later, and probably chat about the experience as well. Meanwhile BA will go through the cabin with a fine tooth comb for possessions which will all be tagged and returned.

Another well known country ......

Lots of people in uniform shouting contradictory instructions at you. Dogs used to keep you in place. Aggressive questioning when you next get to immigration. Denial by staff that anyone could have managed to get into the country without the right stamps. SSSS for life.