All those saying go to the GP, even getting through to some GPs during the day and getting an emergency appointment is a bit hit and miss. If you miss one of the slots, it's missed, whatever you say! That's if you get through.
If the mass was fast growing, hot and looked infected, and you couldn't get to a GP that day, then A and E in the evening might be an option.
Of course, what you have done OP is even better, phone the clinic. I always contact the secretary/clinic numbers and ask for cancellations, as there's bound to be lots, especially in winter/covid.
I just don't think these things are as simple as never go to A and E or go to see GP, it depends on the availability of both, and whether the situation is deteriorating. If you had an infection and were on the way to sepsis, then going to A and E would not be an over-reaction. I sense a tendency on Mumsnet to imagine you have to be dying to go to A and E and at busy times, in some places, that might be the case, but in others, it's not like that, and I don't think people on Mumsnet are equipped to decide if something is life-threatening, untreated infections are life-threatening...
I'll be reminded of this thread next time someone pops up to say life in the Uk is actually fab and we should just all be grateful...if you are not a pushy patient who is prepared to work the system, make extra calls, and just accept what is handed to you, you literally risk dying in the UK, our cancer survival rates indicate that.