Vegetable Oil - now essentially duty free for driving
Legally popping a bottle of vegetable oil requires that a duty of 27p is paid. However, the HMRC recently agreed a de-minimis limit which basically means that people who use under 2,500 litres of vegetable oil a year don't have to pay the duty.
That means that people who pop a bottle of rape seed oil in their [diesel] tank bought in the supermarket are no longer breaking the law.
Even if you convert entirely to vege oil and run at about 20,000 miles a year you now have duty free driving.