Angling is the biggest participation sport in the UK, and by some way, even surpassing football! So there's money to be made.
The rod licence is absolutely nothing to do with the actual fishery you choose to fish at. The rod licence money goes to the environment agency, and they have "officers" patrolling random areas just turning up unannounced at fisheries. You send off for your rod licence at the beginning of each season and they send a little card to you, with a fish on, see post #1. You put it in your tackle box and you don't need to worry about the officers then as you're covered.
Then you choose a place to fish. Usually you start fishing and guys come round collecting the fee from you, which can vary depending on size of lake, popularity of lake and type/size of fish in the water. The fisheries guys don't give a hoot if you have a rod licence or not, it's not their jurisdiction, and the environment guys don't give a hoot if you've paid to fish or not, they're basically separate entities focused only on getting what the angler owes them.
Occasionally, as happened today with me, you'll be fishing and no one will come round to collect the money. So you get a free days fishing, well three hours in my case today. It doesn't happen that often but it's a nice little bonus when it does, lol.