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
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.
This depends as London has 5 airports servicing it. Generally speaking, although probably the furthest away, Luton Airport is the cheapest to fly into and Heathrow would be the most expensive.