[Caff]

I don't know anything about chest pains being a priority - i would assume the nurse that sees you before you go into the queue (in my experience) will prioritise.

I know that a young child will be seen before someone older, for example, and that a 999 call will have been assessed en-route and may well be rushed straight in.

If in doubt about chest pains - I'd call the ambulance service.
The doctor's night-time stand-in might be of help if you're worried about something right now, but if you're really worried - phone for an ambulance.