Perhaps I could have explained more fully...
Matspart3, you are quite correct that the profit on fuel doesn't go anywhere near the cost of maintaining fire cover and running everything else on a licenced airfield. However, I don't think any airfield operator ever intended it to. The reason small GA airfields are licensed is so that flying training can take place there. It is the flying training that covers (more or less) the cost of running the airfield.
Yes, its nice to have fire cover, but this is usually withdrawn when training finishes for the day and the field becomes unlicenced. I haven't seen any waiving of the landing fee because the fire cover has finished for the day.
You will only get a landing clearance at an airfield with a full air traffic service, and even flying training isn't going to cover the cost of that as controllers (quite rightly) are professionals who require an appropriate salary. I don't know of any GA- only airfields with full air traffic service.
The point I'm trying to make (and I perhaps haven't made it very well) is that the base costs of many (most?) GA airfields are covered by the flying training that takes place there. I'm not saying that this a good thing or not a good thing, but that is how it is at present in the UK. Additional business (i.e. the GA visitor flying in for fuel) adds to the bottom line profit of the operation and should be encouraged not discouraged.
If you want to top up the tanks en-route to give you more options in the event of worsening weather, or any number of other in-flight hazards, it really depresses me to have to pay a landing fee to just buy fuel. In some cases it has doubled the cost of the fuel, just so that I have a bigger reserve.
It hits particularly hard at those of us who make longish flights, all year round, in pfa aircraft with limited endurance.
AVGAS is expensive enough (I hope we can all agree on that) and no airfield that I know of sells it at a loss. To discourage additional sales at most GA airfields makes no sense whatever to me.