Só para complementar que o termo "o'clock" é utilizado especificamente para hora cheias. Ou seja horas em ponto. Yes, o´clock is used to mean a given time "on the dot". Ex: The meeting begins at 07:00 o'clock. meaning "The meeting begins on the dot of seven./the meeting begins at seven o´clock on the dot" or even "the meeting begins at 07:00 sharp."
It worth noticing that your tip was about the British way. However, it come in handy, since the learner might learn the two of them (Ame. English and the British one). Mainly, if he is a frequent traveler.
Other than in such services, I mean, in everyday and commom conversation it´s not widely used.
========================================== Military time is an unambiguous, concise method of expressing time used by the military, emergency services (law enforcement, firefighting, paramedics), hospitals, and other entities.