When I was at school (at about the same time as Noah), A 'thank you' for something small required only a smile of acknowledgement in response. 'You're welcome' was regarded as a barbarous Americanism.I was also shocked to find out that your generation of BrE speakers don't acknowledge "thank you" with much either.
In the Czech Republic we don't speak, but it is polite to say 'goodbye' when we leave. I seem to remember that in Germany I used to greet on entering as well as saying goodbye on leaving.You talk to people in the elevator?
If you didn't hear what someone said, do you use "excuse me?" for that?
Going back to the bumping into someone scenario, both parties would tend to say "I'm sorry" in Britain -- one for doing the bumping, and the other for being in their way.