As far as your employment rights go I'm not sure if you have a case as your employer has never given you any guidance on what their policy is anyway. You might have a case if you employer has written a policy stating terms that are different to what you have been told. On the question of tax however from a tax perspective you can claim tax relief on the journey as it is classed as business mileage for tax purposes. Journeys to and from a temporary place of work (this site) are business mileage. Journeys to and from a permanent place of work are commuting and not classed as business mileage. The HMRC approved mileage rate is 40p a mile for the 1st 10,000 miles (when using your own car) , anyone receiving less than this for their business mileage can claim a refund retrospectively for 6 years. You make a claim or work out how much tax relief you are due atcommercial link removed

There is a service to claim back these types of refunds and a guide on commercial link removed There is also a free calculator to work out what your refund might be.