Given a choice between East Coast trains from city centre to city centre or a BA flight from outer London to outer Leeds there is simply no contest.
A much more relaxed journey due to the lack of security, traipsing around etc and any delay is likely to be in the handful of minutes (as Alex found) rather than anything longer. Add to that the frequency of service and it’s a no brainer really.
So you can have a glass of poo at Heathrow. Cheap thrills.