One thing I omitted to mention in my reply to Robert is that in
central London there is now a 'pay-before-you-board' regime for buses
- actually this is something of a misnomer considering that these days
most people already have pre-paid tickets, but for those that don't
they need to buy a ticket (or a one-day bus pass) from the roadside
ticket machine at the bus stop.

Indeed. And London is now usable for ad hoc bus journeys: you can get
on a bus headed in roughly the right direction, slap your Oyster or
wave your day Travelcard, and then look out the window for either
where you're going or an alarming turn off the obvious route.
Contrast the shambles that is Birmingham, where the system is
essentially unusable even for residents: I've used more buses recently
in each of London, New York, Tokyo, all blessed with various pre-pay
systems, than I have in my own city.

ian