If, as an employee, you use your own money for travel or things that you must buy for your job, and you are not reimbursed in full by your employer, you may be able to claim tax relief.
The expenditure must be purely business, and not on anything that you would use in your personal life. You also can’t claim any relief if your employer has provided you with an alternative for these costs.
For example, if you use your own car for business mileage (other than home to work travel),and your employer does not reimburse you at 45p for the first 10,000 miles in a tax year, and 25p for subsequent miles, you should be able to claim tax relief on the underpayment.
If you do wish to make a claim, please see HMRC form P87.
Any claim must be made within 4 years of the end of the tax year in which the expenses arose, and you must keep records of what you have spent.
For further guidance, please see the HMRC website.