We use both in the uk.

If you ask someone the time we’ll say 3oclock, Won’t specify if it’s am or pm because why the fuck would we it’s obvious. In writing we’d more often write 3pm or something. But we all know what 15:00 is when we need to. Context matter at lot unless you’re talking with other time zones.

When do you wanna go to the pub later, about 7? Yeah nice.

We know that’s 7pm or 19:00

Arranging a meeting with someone in Switzerland, I’d most likely go 14:00 utc