Sorry that I missed this before. On this side of the Atlantic, a Do was always a party as you have been told.

It is still used here but tends to be in particular contexts - you always speak about someone's leaving do - rather than a leaving party. There is no reason it is just Custom and Practice like so many things.

The word "do" for party comes up in the "Daily Telegraph" cryptic crossword very regularly. As I am of SanDiego1K generation (although she wears it so much better!), there was a film many many years ago called "Sing as you go" which starred Gracie Fields (no relation to W.C.).

There is a song in the film called

"We're having a Party, We're having a Do" and I bet I am the only FTer to have seen the thing!

A Do tends to be somewhat small in number so calling the London Do - a Do - is quite ridiculous as there are so many people coming that there will be hardly anyone posting that weekend as they will all be careerring around London led by the aforesaid lady like Bouddica in her chariot!