Per diem rates or scale rate payments are fixed amounts paid to an employee to cover business expenses. HMRC issues Benchmark Scale Rates (BSR) and Overseas Scale Rates (OSR).
According to the HMRC, BSR are the maximum amounts an employer can pay or reimburse, free of tax and National Insurance contributions (NCI), without reporting the payments to HMRC under this system.
Choosing to reimburse the actual cost: An employer may choose to pay less. However if a higher amount is paid, the excess is subject to tax and NCI.
So you can either pay the benchmark rates (note that HMRC will require evidence of the expense having incurred but not necessarily the receipts) or reimburse the actual expenditure (with the proper receipts). Usually actual reimbursements are done on occasion where the expenses exceed the benchmark rates.