My understanding which could be wrong but I haven't yet seen anything to contradict me is that in London we have four different methods of boarding depending on the bus route used:
- Boarding by the front door, ensuring you either touch in or show the driver a paper ticket as used by most of London's buses;
- Board by the front door if holding a paper ticket, or any door and touch in if using Oyster as used on the NBfL routes;
- Board by any door with no requirement to present a ticket anywhere as used on the bendy buses, and still used by the bendy-in-spirit-but-not-nature Red Arrow buses;
- Board at the back, and show the conductor a paper ticket or touch in as used on, sadly, only the 15H now.
I admit that what I say about the 507/521 above could be out of date, as it's been a few years since I last caught one. It definitely used to be as I described though, even after de-bendification, the reasoning being that many commuters using them would have paper season tickets (as they'd have arrived from outside London at the major rail termini) which would negate the dual-door boarding if they all had to queue to show the tickets to the driver.