Why do the British say "I need the loo" instead of "I need to use the restroom"?

[Answered on Quora.com by Colin Riegels]

It’s like the old joke about the two ways British men will tell their date that they need to go to the toilet.

A working-class British man will say: “I need to go and take a piss.”

An upper class British man will say: “Will you please excuse me - I just need to go and shake hands with an old friend, whom I hope to be able to introduce you to later.”