Go on the NHS website and book an appointment for tomorrow then, I don't see what could be easier.
Last time I was ill, it was around 10pm and opened the NHS and booked an appointment for the following morning at around 8am, got a phone call from the doctor who asked some questions then said can you come see me within the hour.
Not all GPs sign up to it. Funnily enough I can't do it for mine.