The right of patients to receive treatment free at the point of delivery is a GMS contractual obligation.  The relevant paragraph is this

14.2.2. Subject to clause 14.2.4 and 14.2.5 and to clauses 14.6 to 14.7 a prescriber shall order any drugs, medicines or appliances which are needed for the treatment of any patient who is receiving treatment under the Contract by— (a) issuing to that patient a non-electronic prescription form or nonelectronic repeatable prescription completed in accordance with clause 14.2.8; or (b) where clause 14.3 applies, creating and transmitting an electronic prescription.

GPs can advise patients that treatments are available without prescription, but were a GP to refuse to issue an FP10 for treatment that they had recommended they would be in breach of paragraph 14.2.2 and open to complaint and possible financial redress.