Munnyspinner
As said by others it's clearly marked at numerous stages of the booking/check-in process. Conformance has proved hugely beneficial to BA's punctuality levels as it means they know 35 mins before departure if they're going to have to offload bags rather than waiting until the gate closes to see who didn't make it. You have to draw the line somewhere. If they decided to cut off at 30 mins they'd be good in your books but someone else who turned up at 29 mins would want to be let through. While it may prove annoying to a few people like you who don't read the instructions, for the majority of passengers who've turned up with a reasonable amount of time to spare it means they can leave on time or even early.
Your claim that bmi is more punctual than BA is wrong, since T5 (and largely due to conformance) BA is now much more punctual. They are also a lot less likely to lose your bag than bmi.
It's up to you who you travel with. If you don't like the conformance just fly bmi. Although the latest bmi wheeze is they're replacing more and more LHR routes with E-135/145s now, so I hope you don't mind cramped spaces...