Some airfields, like Old Sarum, offer monthly landing cards for about £25. Buy one of these and you are free to use the airfield as much as you want for the month. You could do circuits until the cows come home.
We normally buy one for our group as it is nice to be able to pop in and do some circuits, or have lunch for no additional cost.
I believe Compton Abbas do this too, though I hear strange things about CA at the moment.