One might have a course, broadcast by the OU, followed by an exam. The course would not turn you into a fully qualified pharmacist, but would cover the areas of technical knowledge, ethical and social issues you need to be a well-informed user. If you pass you would get a credit-card like licence which enable you to buy controlled drugs, at the market price, from a pharmacist, without a prescription. Your purchase might be registered in order to restrict you to buying personal quantities and to impede any subsequent criminal use of the drugs. If you were caught giving or selling the drugs to anyone else, you would lose the licence in addition to suffering the due penalties.
Authority is incompatible with offering a service.
Doctors are also the product of a state-based university system that promotes an abuse culture and has values entirely incompatible with the online society. More mundanely, their services are shaped by the demands of the average uk citizen, whose interests are alien to my own recreational - I do not use drugs recreationally myself, except alcohol, but would like to be at liberty to do so. There is no virtue in abstinence if it is mandatory. I do not want to be searched for drugs. Again I might well wish to seek advice from a doctor, but it would be a gross violation of my privacy to require this.