I've heard the term used in London as well. But never in the US. I first ran across it when I lived in Australia, so I wonder if it's a British term.

It's possible it's an Australian term and you were just surrounded by Australians in London.

Yes. I've never heard anyone use it in the UK except Aussies and Kiwis.

I've heard it used in wonky circles, and if you pronounce the whole thing instead of the acronym people know what you mean.

New York City is unusual for having two different CBDs, with midtown being denser than downtown (but each are denser than anywhere else in the US).

Houston uses it occasionally. Probably picked it up from all the oil expats in town.