Ha, yeah good spot. In the trades (builders, joiners, plumbers, electrician etc) everything is done in mm in the UK. I’m not a tradesperson (degrees are in electrical and electronic engineering so equally happy talking in nano and micro metres as I am anything else) but have renovated houses and now run a factory that I spec’d, manufacturing a physical product that I co-designed and mm is now my unit of choice. I was measuring the internals of a van I am converting into a camper the other week and wrote everything in mm, even the length at 4050mm!

My assumption is that it is a. more accurate and b. allows you to talk about a 22mm pipe (standard plumbing diameter pipe) in the same unit as a 2400x1200 sheet of plaster board, without having to worry about what to do with the decimal point - pure speculation though.

Ironically people will say “twelve hundred” in this context which is deemed to be of American influence, but in any other context they would more likely say “one thousand two hundred”. Given that the US is imperial this can’t be a trade influence - must be for speed and ease of pronunciation.