Quick followup, and apologies if I missed the answer on this (very thorough) thread.
On a YYZ-LHR-FRA flight, I fly both in and out of terminal 2, AC in and LH out. Based on
these instructions from the Heathrow website, there is no difference between European and non-European departures for *A? I understand Britain is not part of the Schengen zone, so I'm guessing this is the reason.
I'm thinking, if for whatever (really remote) reason they decided to join the Schengen zone in the future, they would have to somehow "split" Terminal 2 up for security purposes?