To spend a penny, is a common British idiom which means: to go to the toilet.
According to my grandparents, the word loo and toilet were vulgar, and the expression:’ go to the restroom’ was completely inaccurate, consequently they used to say. ‘Please forgive me, but I must go and spend a penny’.
I know?!? It wasn’t until I was in my teens that I understood what they were talking about. However, more importantly, it costs 30 pennies nowadays. What a rip off!
As an aside, in Vietnam, the loo is called The Happy Room and in the Philippines it is called a Comfort Room. I’m not saying anything, BUT, my definition of comfort is definitely not the same as it is in the Philippines.