Hello there, @callumpearson28.
Allow me to provide some insights about Mileage Allowance.
An employee or business owner who uses his car for business travel can claim mileage at the relevant amount. For a car, it’s 45p up to 10,000 miles. If your employer pays less than these rates, you may be eligible to claim tax relief.
You can check out this article on how to calculate the allowable business expense for your car based on the mileage you drive: Car, van, and travel expenses.
For more information, you can also visit https://www.gov.uk/.
If there's anything else that I can help you with, please let me know in the comment section down below. I'll be always around ready to help.