T5.
Delta uses Heathrow terminal T4.
T3.
There are various places you can fly to from Heathrow Terminal 3 such as Chicago USA, Amman, Oslo and Vancouver Canada. It is an International terminal that help you in your travel destination.
Numerous airlines fly from Heathrow to Austrailia, from different terminals.
Yes
Delta fly in and out of Terminal 4 at London Heathrow. See related link for more information.
Terminal 3
9 hours 45 minutes NON Stop on British Airways from Phoenix to London Heathrow Airport
Thai Airways operates from Terminal 2 at Heathrow Airport. This terminal serves as the home for various international airlines and is known for its modern amenities and facilities. Passengers flying with Thai Airways can enjoy a range of services before their departure.
Most would fly into either London Heathrow or Gatwick Airport.
British Airways (BA) typically flies into Terminal 5 at London Heathrow Airport (LHR) from Dublin Airport (DUB). However, it's always best to check your flight details prior to departure, as terminal assignments can change.
That info would come from the airline.