I am traveling from NYC to Mumbai via London. Assume my connecting flight belongs to different terminal. If I have to change terminal at Heathrow, do I need to go through UK Border control ?
No, there is no need to pass through immigration control in this circumstance.
Simply follow the purple signs to the new terminal. The inter-terminal bus system is before immigration control.
If you are flying BA then you are in Terminal 5 for both flights, and there will be no need to use the buses.
You may need cross the border control. As per https://www.gov.uk/check-uk-visa
Below is the excerpt
Will you pass through UK border control?
You always pass through border control if you:
- leave the main airport building for any reason
- need to collect your bags and check them in to your onward flight
You must also pass through border control if both:
- your onward flight leaves on a different calendar day to when you arrive
- there’s nowhere for you to stay overnight in the airport, for
example in a transit hotel
Check with your airline if you’re not sure if you’ll pass through border control.