A valid logbook covers a continuous, representative twelve weeks, records each business trip with its odometer readings and a specific purpose, and then holds for up to five years. Running costs include fuel, servicing, registration and insurance, plus deemed depreciation and interest for owned cars or the lease charges for leased ones.
High business use can make this method much cheaper than the statutory formula. The catch is keeping a valid logbook and classifying trips correctly. Telematics can make that easier.
General information, not tax or legal advice.