It does not really matter where they depart - because that can change. The terminal information is only relevant for you to see where you will find the check in for your airline. BA for example does fly out of both Terminals. Once you are airside, you can walk between both sides easily, but it might take some minutes.

The airport lounge is also accessible by oneworld though it depends where your flight is going from - no good idea to go into this Terminal 2 lounge and fly from teh end of Terminal 1 and vice versa.