In everyday use, UTC and GMT display the same hours and minutes. The difference is what they are.
GMT is a time zone based on the sun's position at the Greenwich meridian. UTC is a standard maintained by atomic clocks, kept close to the earth's rotation. UTC is the definition; GMT is a zone that happens to sit at the same offset.
Which should you write?
For anything technical — logs, APIs, flight times, international schedules — write UTC. For casual British context, GMT is understood, but remember it is wrong for half the year in the UK itself.