The amount that can be paid for business mileage is determined by HMRC. There are 2 sets of rules, depending on whether you have a company car or receive a car allowance, or neither.
If you don’t have a car or allowance, HMRC says you can claim 45p per mile for up to 10,000 miles, and then 25p per mile. That figure has remained the same for a long time.
If you do have a car allowance, HMRC’s rate is updated every 3 months (1st March, June, September, and December), and the figure in the Expenses Policy is updated accordingly. You can find the HMRC’s calculations at https://www.gov.uk/guidance/advisory-fuel-rates#how-rates-are-calculated. These rates are also used for reimbursing the Society for personal mileage if you have a company car.