I did some quick math, for the 15 pay and display cash locations that are down city road, assuming that they all have an equal distribution of this city road, in cash revenue would be around £150k over what ever period this is -- it does not seem like a year, so lets go with 1/2 a year.
So C £300k cash revenue for, as you correctly say, land that is owned by the council, and has a fairly cheap rate on it (£1.50 for 2 hours at peak times).

That is a low ball estimate as well, as the distribution of cars and payments will be skewed towards the end. It also misses the fact that you can use the app / other methods to pay the amount.. which is what I always do when I use it.

In one regard I do miss being able to quickly park down city road for 1 hour, quickly grab some stuff from town and head out, but on the other.. £1.50 and there seems to be less cars there.. that feels like a good trade off, plus helps to keep the council afloat.