Check the airport’s official terminal finder — assignments follow the airline, and sometimes the route. Broadly: British Airways and Iberia fly from Terminal 5; Virgin Atlantic, American, Delta, Emirates, Cathay Pacific and Qantas from Terminal 3; Qatar Airways, KLM, Air France, Etihad and Korean Air from Terminal 4; Lufthansa, United, Singapore Airlines, Air Canada and Turkish from Terminal 2. British Airways is the one airline with flights out of two terminals (T5 plus some T3 departures), so confirm your specific flight.