Because you are essentially an employee, any expenses will be paid in line with your umbrella employer’s expenses policy.
Many employers’ expenses policies look at whether an employee is subject to supervision, direction and control or not. If you are not subject to supervision, direction and control in the manner of how you work, then you may be able to claim mileage, car parking costs, HMRC incidental expenses costs and training intrinsic to your role via the payroll. You can only do this if they are ‘wholly, exclusively and necessary’ in the performance of your role. Any other expenses which are wholly, exclusively and necessary to the performance of your role can be claimed via your self -assessment return or a P87 Form.
However, if your end hirer reimburses you for expenses which are over and above your normal daily or hourly pay rate, these expenses may benefit from tax relief via the payroll if they are not part of your normal commute. In other words, if they relate to ad hoc visits to other work sites.
If you are a mobile worker you should also be able to benefit from tax relief via the payroll. It’s worth noting though that expenses incurred solely through your work can still be claimed on a tax return through HMRC, for example, if you have to pay for protective clothing or subscriptions necessary to your role.