What are the rules on prescribing several months worth of prescription if you are travelling?

I am travelling for 3 months. I rang my pharmacist, and they said they can legally only dispense one month at a time. This seems odd, and bizarre. Is this true?

I spoke to Psychiatry UK to ask if they would do it. They said they would, but they cannot, because I already have a shared care agreement with my GP.

Anyone have any advice?