I think the Customs people only got funny if the fuel was used in something other than an aeroplane. It was, if I recall correctly, delivered "duty free and unmarked(i.e. no dye colouring) for aircraft engine use only". We had to be careful how much was put into the road diesel to stop it from 'waxing' during very cold weather, otherwise the revenue boys were on our tail!
Refuelling civvy aicraft did have its drawbacks - MoD were chasing Jack Nicklaus for months for payment for the fuel he uplifted at Leuchars when he flew his private jet -N1JN- in for the 1978 Open at St Andrews. It wasn't as if he was short of a bob or two either - he won the event!
Everytime I see 558 , I say a quiet 'thank you' to those who stopped her from going for scrap.