So basically if you have a car that is 1300cc petrol and you have done 8000 miles you can claim VAT of
8000 x 14p x 1/6 = £186.67.
You would need to retain fuel receipts to ensure you have incurred at least that amount of VAT ie receipts worth £1120 (8000 x 14p).
The £186.67 would be deducted from the mileage you put through the accounts which if based on 45p per mile will be £3600 (8000 x 45p) so would show £3413.33 in the p&l (£3600-£186.67) rather than £3600 (gross).