Don’t forget to claim the VAT on business mileage.
You can’t claim VAT on the full 45p but you can claim on the fuel element. You can find out the current fuel element Advisory fuel rates – GOV.UK (www.gov.uk)
For instance my care has a fuel rate of 14p. This means that, for every mile, I can claim 14p *20/120 = 2.33p in VAT. There is no VAT on the remaining 31p as that is deemed to be for insurance, maintenance, wear and tear etc.
All you need to do is to keep a VAT invoice* (not a credit card receipt please!) for fuel around the date of the journey ie when you you fill up before or after the trip.
*If you’re interested then it’s because the 45p is an allowance and EU/UK law states that you can’t claim VAT on allowances. The EU did one of their usual accommodating moves and agreed that, as long as there was a valid VAT invoice for fuel around the same date (eg filling up before or after the mileage) then the company could claim VAT on the fuel element of the 45p.