Claiming luggage (but there is an airside luggage check in Terminal 5
if you have additional luggage to check in London)
Changing airport
Flight is cancelled/delayed and you have to leave because the airport
is closing for the night (there is a procedure for this, you will be
allowed out)
Getting arrested (but the border formalities will be skipped, I think, in this instance)
As I said in your other question, you can move between terminals freely without crossing the UK border. You can ride the buses all day if you like.