Originally Posted by
frappant
Aim to arrive at the airport between 10 and 10:30, which would mean getting on the Underground by around 9:30.
I would tend to arrive around 10:30 for a 1PM flight out of LHR. Boarding would start around 12:15pm.
I dislike feeling stressed about timing, especially before an Intl flight, and that gives plenty of time for some drinks and food at the United Club beforehand.
Also allows a nice cushion in case of issues getting to airport, etc. Which in London, is not a rare occurrence.