If you opt to use the simpler 'cash basis' method of submitting your tax return, you don't need to provide a detailed breakdown of your expenses at all. You simply tell HMRC, for example,
"Gross Income (£51,234) - Expenses (£29,876) = Taxable Income (£21,358)". That's all there is to it!
Of course you need to be able justify your figures if the tax man asks you for further information but I've never had to. So, when doing your own calculations, put down whatever you've genuinely spent on fuel and only worry about proving it if Mr Tax Man comes a-calling (which he almost certainly won't anyway).
https://www.gov.uk/simpler-income-tax-cash-basis