The idea being that if you want to take a trip to somewhere tropical, you don't have to tie up a GP with a standard, low-risk medication, but it's not quite so low-risk that you just put it on the shelf. Behind-the-counter, if you will.

https://en.wikipedia.org/wiki/Over-the-counter_drug#United_K...

> Pharmacy Medicines (P) are medicines which are legally neither a POM or GSL medication. These can be sold from a registered pharmacy but should not be available for self-selection (although directions to discuss a 'P' product may be allocated shelf space with associated GSL items). 'P' medications are reserved from the GSL list as they are either associated with a need for advice on use, or used in conditions which may require referral to a medical prescriber. Suitable trained counter assistants may sell a 'P' medication under the supervision of a pharmacist and will ask questions to determine if the customer needs to be referred for a discussion with a pharmacist.