Be as accurate as you can be because if you drive more than the mileage you state at the beginning of the rental term, you’ll be charged.
The cost is usually 10p per mile driven over the stated maximum. So if you drive an extra 1,000 miles per year, and your contract is for three years, so you do 3,000 more miles in total, you will need to pay 3,000 x 10p = £300.
If you’re borderline, we’d recommend you increase your expected mileage, so you know exactly what costs are coming your way.