IO540,
The boys & gals in blue have enough to do without opening at weekends for a couple of puddle-jumpers. Most RAF airfields do not hold AVGAS and it would be ludicrously inefficient to open an airfield for one light aircraft. £10-£20 wouldn't even cover the cost of lighting let alone all the other bits and pieces. Make no mistake, you cannot compare a civilian airfield with a military one.