Harry Bloomfield wrote:

At the lower edge front and at the rear of my photo licence, it lists
the vehicles I am qualified to drive, in capital letters, on the front
after the capital letters, it lists several more in lower case.
Likewise, on the rear, it lists those same lowercase letters f/k/l/n/p/q

What are they?

https://www.gov.uk/old-driving-licence-categories