An employee (or business owner) who uses their own car for business travel can claim mileage at the relevant amount (for a car it's currently 45p up to 10,000 miles and 25p thereafter).
HMRC say that the 45p per mile (or 25p) that you claim for using a car, not only covers petrol but it also covers wear and tear and other running costs. As well, you can claim VAT on the petrol element of the 45p/25p.
For the petrol element, you can use the advisory rates published by HMRC.
Practically speaking this would like like the following.
Presuming you have a petrol car with an engine size (cc) of up to 1,400, the advisory fuel rate is 12p.
The formula would be as following
Advisory fuel rate * number of miles = £X (inclusive of standard rated VAT)
(45p less advisory fuel rate) * number of miles = £X (Inclusive of no VAT)