Re: Oh what sillyness
"When you buy petrol at a garage it is of course taxed, whether you put it in a car or an aircraft. AVGAS too is taxed (and is more expensive than petrol), as I believe is AVTUR (kerosene) used for private flying."
A quick scan over https://www.gov.uk/fuel-duty and following the links on duty relief seem to indicate that AVGAS is 20p/l less than petrol and AVTUR is the same duty as petrol but for commercial use, both AVGAS and AVTUR are duty free.