When a union negotiates pay rates across a sector for a particular group they are negotiating the pay rates for the individual who makes the parts/ provides the service etc. They are not negotiating the cost of those goods or services to the consumer (although they will have some effect on that cost).
The fee a doctor charges is not the same as their income as they will have varying levels of cost associated with their medical and public liability insurance, rent or mortgage on their premises, staff wages etc.
While unions are perfectly entitled to negotiate the rates of pay for the receptionist, nurse and employed locum doctor within a GP practice I think their has to be a line drawn when they fix the charges that the GP or GPs who own the practice charge as employers.
In my innocence I thought that unions were there to represent employees, not business owners.