It’s not just VAT, there are other reasons why you need good contractual documentation showing that your role is just acting as an agent in introducing the patient and dentist. For example, what would happen if the patient was unhappy with the treatment – or the dentist did something wrong? Without any documentation proving otherwise, the assumption may be that you’re the supplier of dental services to the patient – making you potentially liable for any claims.
Also in the documentation – make it clear who you’re acting as agent for. I.e. if you’re the patients’ agent, they are receiving both a supply of treatment from the dentist and a supply of introduction/booking services from you. But if you’re the dentists’ agent, the patients’ are only receiving a supply of treatment from the dentist, and the dentist receives the supply of the introduction/booking service from you. The only impact of this is on who you would need to issue your VAT invoice/receipt to. As dentists’ supplies are predominantly exempt, it is unlikely that they would be VAT registered, so would be unable to reclaim any of the VAT they are charged.
I actually believe that if you were regarded as the supplier of the dental services to the patient (i.e. you were acting as principal), your whole charge to the patient would be VAT exempt – you wouldn’t need to charge VAT on any of it. In the VAT Act 1994, schedule 9, group 7, item 2 exempts treatment by registered dentists – but note 2 to the same group exempts supplies by those without the appropriate medical registrations, provided that the actual work is carried out by a registered person. So just from a VAT perspective alone, you may be better off if you weren’t just acting as an agent. But the risks of being responsible for the dentists’ work would probably outweigh the VAT benefit – it’s safer to make sure that you’re just the agent, and declare VAT on the commission element.