If I am traveling from NY to Mumbai, via London, what are all the possibilities/reasons that may cause me to go through UK border control? Eg: Changing terminal? Eg: Rechecking luggage? Anything else?
- 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.