I don't buy the idea that it's about avoiding waste because a prescription can be dispensed in installments.
For example my dad takes oxycodone which is a controlled drug and the surgery refuse to let anyone have more that 4 weeks at a time.
It was very stressful and he was constantly running out, so I asked them if they would send a prescription for 8 weeks to the pharmacy, with instructions to "dispense 4 weekly".
Now, I only have to chase up the prescription at the surgery every 8 weeks which is massively time saving for all concerned, and the pharmacy deliver the second 4 week installment when it's due without me even having to phone them.
Additionally the biggest cost to the NHS is clinician time, not a pack of medication that costs a couple of pounds or even pennies in some instances.
Think what the GP, pharmacist and administrative staff involved in processing your request are all being paid hourly - turning over prescriptions on a large scale every 4 weeks is a colossal waste of the NHS's most precious resource.
The health board next to mine where my in-laws live have a policy of GP's sending 12 MONTHS of long term prescriptions to the pharmacy, and the patient collects a month at a time. It's brilliant and very little medication is wasted.