Why is it that whenever I go in a charity shop in the UK, someone is training someone else how to use the cash register? Is this a purely British phenomenon, or does the same thing happen in US thrift shops?

Here in the States, these places (most stores in general) have rather large employee turn over. It does seem like they are frequently training new people.

Interesting observation! My guess is that many charity shops hire volunteers, many of whom are probably young people motivated by the idea of getting some experience under their belt while they're on the hunt for paid retail jobs.