You can claim mileage for any wholly and exclusively business travel in your car (45p per mile), your motorbike (24p per mile) and on your bike (20p per mile). The car rate reduces to 25p per mile for any mileage over 10,000 miles in a tax year. There are a number of apps out there that can automatically calculate your mileage (TripCatcher and MileIQ are two of them, but also several others), but this is one of the few occasions we to like to use a spreadsheet (app fatigue anyone?) . We are happy to share this with anyone who needs it. The most important thing for mileage is to ensure you log three things: date, purpose of the journey, and number of miles. Without these three pieces of information, HMRC are likely to reject a claim in the event of an investigation. We recommend building the claim over a month, and then processing it into the accounts on a monthly basis. If you have a lot of mileage, you may wish to do this more regularly.
To actually claim the mileage back, you have a choice: actually withdraw the money from the business bank account each month (or whenever you submit the claim), or leave it until the end of the financial or tax year and adjust your profits accordingly. We usually recommend doing it as part of the process at the end of the year.