A consultation is £40 for 20 minutes. That is the same price whether you are coming for a general problem, a skin complaint, a medical letter or a weight management review — no premium services, and no working your way up through appointment types.
Twenty minutes is the point. A standard NHS appointment is ten, and most private online services work to similar lengths. Doubling it is what makes it possible to take a proper history rather than deal with the first thing you mention and run out of time.
If you expect to need a GP more than a few times a year, membership covers unlimited consultations from £15 a month, with a family option. It is entirely optional — nothing is held behind it, and pay-as-you-go is cheaper for most people.