I see at least some people saying hello and thanks almost everywhere, but it's rarely consistent. In London, it's fairly common, at least in the outer boroughs, to see people saying thanks if they get off at the front. If I'm exiting in the middle/rear, I'm not going to shout to the driver unless the bus is empty, though I may wave if I see they're looking. If they're nice enough to wait for me while I run down the street to catch the bus, *of course* I thank them.

Time of day makes a difference, too. I've had occasion to ride the buses late at night to/from Heathrow, and then you'll see people who ride the same bus every night and know the drivers. They're sometimes quite chatty with one another, and I've seen that lots of other places at less-busy times.