I assume that, as in the US, SI units are used in many scientific and technical contexts.

It’s kilometres specifically that I hear used rarely in the Uk. Other SI units are relatively common I agree.

People who are into running/cycling use "K" a lot, personally I think it's because once someone starts talking in km, if you use miles your stats will look bad by comparison. I know trackers/apps probably had this as default from the start because Europe=metric but so did a lot of satnavs and it didn't change our language in the same way.

mm is used constantly in engineering and km will be used for long distances in professional settings.

Metres and mm I hear a lot but kilometres very little - mainly because of miles on road signs. I guess the last reference I remember is to 5K runs.

It's rare to need kilometres in engineering, but I know from a friend that works at Network Rail that everything is done in kilometres, except final conversions on many older railway lines where the mileposts still measure miles.

Those small blue signs at the side of motorways, and the white emergency location posts, both use kilometres.

>It's rare to need kilometres in engineering

Yeah, the thing with imperial units and engineering is that once you get anywhere near mass, weight, force, anergy, and all the related units for same, it can get very confusing and error prone. So are we talking pounds-mass, pounds-force, or slugs? (The latter being something that probably no one who didn't take engineering/physics/etc. classes has ever heard of.)

It's incredibly common to use km here, and miles.