Ive been charging customers for mileage @ 45p per mile as a Task labelled Travel on the invoice
As im a sole trader i am using my own vehicle (purchased via the business) and i purchase fuel through the business for business mileage only
The 45p/mile that you are charging your customers is just income the same as any other fees or charges on your invoices - it’s part of your taxable turnover.
Independent of that you will be accounting somehow for the costs to your business of running a vehicle, whether that’s by using HMRC allowances for business mileage (45p/mile for the first 10k miles, 25p/mile beyond that) or by recording all mileage - business and private - and then claiming the business proportion of your actual costs. I presume the latter as you talk about the vehicle being “purchased via the business”.
The charges to your customers increase your income, the claimed costs or allowances increase your expenditure, so they’ll more or less cancel out on the bottom line.