The OP's situation sounds unusual - and it clearly wasn't a distance issue since AA leaves from the middle of the airport so only had half a runway's length to taxi. It was probably runway congestion or, more likely, transatlantic slot congestion.

IME LGW can have very long wait times at the end of the runway as it's an incredibly congested single runway operation. In terms of distance, the longest I've had was at CDG where it must have been at least 15 minutes of quite fast taxiing.