Spending a penny refers to the fact that you have to pay to use many public toilets in the U.K. and for the longest time it was literally a penny.

This resulted in the act of civil disobedience of holding stall door opens for the next person, but paying for toilets is still a very common form of hostile architecture.

I'm fond of another phrase that Americans (and even some Brits don't get). "It's black over Bill's mothers."

Means?

It's going to rain.