[lucozade]

In UK train stations this is so that they can pay for attendants in the lavatories.

When I was growing up this wasn't the case and they were the hangout of some, ahem, undesirable folk. You most definitely wouldn't have sent a child down on their own. They'd end up with an audience...

[chimprich]

Toilets in most UK stations are free. Only in London and some other larger cities have they adopted this anti-social practice.

[LukeB_UK]

They're usually free once you're past the ticket barriers in the cities.