From www.dieselveg.com

Maximum allowable veg oil usage in Litres before having to register or pay fuel duty to UK Revenue & Customs is set at 2500 Litres Per Year, below you can see how many miles or kilometres this equates to when compared to your vehicles Miles Per Gallon

Miles you can run on veg oil before you have to
register or pay duty:____10,998_13,748_16,498_19,247_21,997_24,747 _27,496_30,246_32,995
Average Miles Per Gallon:____20_____25_____30_____35_____40_____45__ ___50_____55_____60

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For the average 10,000 miles per year driver you shouldn't need to pay duty it seems.