In the UK anything "serious" like a train/plane ticket/timetable uses the 24 hour clock. That includes the default way to show a digital clock on a watch, phone or computer.

Timetables yes, but whenever I've bought a digital watch or set up a new device, the default has always been 12-hour. If you ask people the time in the afternoon, they will almost always give you the 12-hour format. People can understand both, but typically default to 12-hour times.

This is in stark constrast to Germany, say, where people colloquially use 24-hour times, with some exceptions for round times (e.g. 17:00 might be called "um fünf", but 17:05 would usually be described as "siebzehn uhr fünf", roughly translated as "seventeen oh five".

This might have changed in the last five years or so since I was living in the UK, but I've never noticed this be different when I was visiting, nor when speaking to British friends or colleagues.

Not "serious" per se, but anything which can be subject to misunderstanding.