Back to the topic, I can't see a rational basis for saying £20 is taking the micky. Sure, it could be lower, but you probably paid £100-200 to get there, and the difference to the airfield is huge (a factor of two actually
).
Experience around the UK tells us that £20 delivers a much better maintained place than £10. Obviously much of it is down to management etc but £10 is evidently hard to make things work at.
Private strips can be free, etc, and there is a vast pilot community which will absolutely not pay more than £5 (they must fly mainly between strips) but then somebody is subsidising it by cutting the grass etc in their own time, which is lovely but not substainable.
I can quote one grass strip next to a golf course in the SW where the landing fee is £20. It has no facilities at all but charges the same as Gloucester.
Another charges the same as a local regional spaceport. But here it is because of stringent restrictions on movements by the local authority and although the owner would love to welcome visitors, the local Nimbys are watching for a breach in the agreement to have the airfield closed.