Sundae, if you're expecting a few of these prescriptions,. might be worth getting a 3 month prepay certificate. They cost about £28 I think. Unlimited use during the cover period. Can do it online and set the date you want it to start.
[eta] Here's a link:
https://apps.nhsbsa.nhs.uk/ppcwebsales/
Don't actually need your NHS number btw to apply, it's just an optional. Costs £29.50 for 3 months.