Heathrow terminals by airline
Terminal 2 (The Queen's Terminal)
Lufthansa, Swiss, Austrian, Brussels Airlines, United, Air Canada, Singapore Airlines, ANA, Turkish Airlines, Air India, Ethiopian, plus Aer Lingus and TAP Air Portugal.
Terminal 3
Virgin Atlantic, Delta, American Airlines, Cathay Pacific, Qantas, Japan Airlines, Finnair, plus Emirates (some flights). Long-haul leisure and premium carriers.
Terminal 4
Air France, KLM, Korean Air, Etihad, Qatar Airways, Gulf Air, Saudia, Malaysia Airlines, El Al, Kuwait Airways and most other Middle East and Asia carriers.
Terminal 5
British Airways (the large majority of BA flights) and Iberia. A handful of BA flights use T3, so check your boarding pass.
Getting between Heathrow terminals
Terminals 2 and 3 sit together in the Central Terminal Area and are a short covered walk apart. Terminal 4 and Terminal 5 are separate satellites. To move between them, use the free inter-terminal Heathrow Express train or the free transfer buses. Allow 20 to 30 minutes between Terminal 5 and Terminal 4, the two furthest apart.
If you are connecting between flights at different terminals, build in this transfer time on top of your minimum connection time. If you are arriving by taxi from Aylesbury, your driver takes you straight to the correct terminal so there is no transfer needed.