> Isn't this a well-known cultural difference between British and... just about any other culture?

At least in French, litotes are meant to amplify the negative.

The classical example being in Le Cid, "Go, I don't hate you" from Chimène to Rodrigue (who just killed her father), meaning "I love you".

Other example can be found, "Not OK" meaning totally unacceptable, "Not bad" meaning excellent, or "Not stupid" meaning very clever. Of course, all litotes can be interpreted literally, and you often need context or tone to convey the meaning