The Flagship Check-in can be used by passengers in F, J as well as EXPs. I have never found it to be any quicker or slower than the main terminal. It is a nice environment, but further from the HEX and tube exit.

PLTs can use the first class check-in desk, but this can be slower than the business class check-in because it often has only one agent working as opposed to three at the business class check-in and so if you get a passenger in the queue ahead of you at the first class check-in who is doing something complicated, you may be waiting a few minutes.