no matter, you are where you should be! Whether it's terminal 1, 2 or 3 you'll pass through passport control and customs. If you leave the aircraft in the first group of passengers you should expect to get through immigration fairly quickly but allow yourself anything from 10 mins to 1hour.
How long should I arrive at Heathrow before an international flight?
three hoursWhen to arrive. We recommend getting to the airport three hours before your flight if you're travelling internationally, or two hours if you're travelling domestically or to Europe.