The 45/25pence per mile is the HMRC approved rate payable to an employee before it triggers the requirement to report for tax consideration.
https://www.gov.uk/expenses-and-benefit ... es-for-taxA business can charge whatever mileage rate it wishes to a customer and account for it (and the associated expenses if it is a company vehicle) within the company accounts.
If the OP is self employed and would normally use a company vehicle for work travel (as opposed to an employee using his own private vehicle, for which the HMRC approved rate can be paid without tax consideration), his company can charge it out as part of the invoice, but in this case will see income but incur no expenses to offset against tax for that particular journey and hence be potentially liable for tax on that income.
Still nicer than driving though.....