timetable for LHR T5 to LGW and vice versa is 60 minutes and more often in reality it’s even quicker using National Express not an hour and a half.

BA won’t sell a ticket with less than a 3 hours connection.- even allowing for the bag drop deadline of an hour long and 45 minutes shorthaul. If your BA flight is late BA will rebook you if on a single ticket. Note on separate tickets it won’t do anything for you.

if lots of people missed their flights because 3 hours wasn’t enough then BA would soon increase the time required. But they haven’t so are happy to bear the costs of the few that don’t make it.