Until recently, HMRC allowed taxable travel expenses to be included in public sector PSAs in respect of normal commuting costs for Non-Executive Directors (NED). The Department of Business (BEIS) wrote to the bodies it oversees on 30 May 2019 instructing them that any payments for commuting made to non-executives and other office holders, will now have to be paid through payroll, with tax and National Insurance deducted at source.
Note also that fees for NED roles in the public and private sectors are always required to be subject to tax and NI through the payroll, as this is income for the holding of an office so it cannot be invoiced and paid gross to the NED.