Incidentally, there is data out there that says that BA long-hauls out of conformance-less T3 have a higher punctuality than BA long-hauls out of T5.
Love BA onboard service, hate connecting at T5. Especially when you're stopped at conformance at T-35, only to see at T-15 that the aircraft you've been offloaded from by the conformance BS is delayed by 45 minutes and you would have had ample of time to make it, instead of being stuck with a 3-hour delay.