If you reward your staff based on their performance, then these must go through payroll and a P11D!
Step 1 – Payroll
You must add the value of the voucher to the payroll as a calculation only so that they pay the relevant national insurance on these vouchers. These should be added at the time the voucher was provided.
Step 2 – P11D
At the end of the year, you should add these vouchers to the P11D so that HMRC can adjust their tax needed to pay the tax on these vouchers. Please note that the value of the vouchers will not be added to the P11DB as the national insurance due has already been paid through the payroll.
If you are rewarding your staff for their birthday or just a gesture of goodwill, then these can be treated as trivial benefits if they are under the value of £50 but they must not be for performance related reasons.