My own thoughts are that this is a very good idea and consider it to be perfectly reasonable. I fail to understand how anybody can honestly think they will be able to arrive at the gate with suffcient time to allow for the completion of boarding the entire aircraft and for all other processes that go unseen to be finished, if the passenger cannot even deign to be at security 35 minutes prior to scheduled departure. We are talking about planes here, not trains.

Nobody is that important anyway (and if are, they aren't flying through T5). In any case, important or not, planning one's diary in a way that allows less than 35 minutes to get from ground transport to gate, especially at an airport as massive as LHR, is completely naïve and unworkable.

There has to be a boundary set, and it has to be adhered to. T5 is a big place, so 35 minutes seems a good measure. You can't make exceptions for people without hold baggage, for frequent travelers, for Goldcard holders. It has to be kept simple and easy to understand. Making exceptions or using experienced travelers as the standard only will complicate an otherwise simple system.

I also question how much of a problem this will really be... I don't believe there are so many passengers needing to arrive at the airport less than 35 minutes that should they be refused boarding, BA would feel pain in their bottom line.