Hi Yvonne,
Under the Veterinary Medicines Directorate, which controls veterinary drug use, a vet must first carry out a clinical examination of an animal before s/he can prescribe any drugs, and the animal in question must be under the vets care - i.e. a registered patient. Also, only the minimum amount of drug required for treatment can be given. It is a criminal offence if the vet precribes otherwise!
For a repeat prescription, the vet must have seen the animal recently enough to have knowledge of the animals condition. Exactly how recent depends on the patient, client, the condition of the animal and medicine being prescribed, and is down to professional judgement on the part of the vet.
Some practices adopt this legislation into practice rules so everyone is 'singing off the same hymn sheet' and working by the same rules - in your case, that the animal has to be seen every three months for repeat prescriptions to continue. In other practices, this may be different.
Hope this helps!