I was told the same thing, it's all about budgets. It's surgery is giving a certain amount of money by the trust and that forms their budget. If they use more on salaries then they have less for cleaning, suppies, admin, etc, but they can get more money based on the number of Rx they write.
Each month I have to schlep over to drop off a renewal for my allergy meds, they take 2 days to write a new one, and then I go back and collect it. It's such a drama...I tried to get 6 months at a go and they won't do it.