Income: Employment income rules: The cash equivalent of any cash voucher received by the claimant chargeable to income tax under section 81 of ITEPA 2003
The Tax Credits (Definition and Calculation of Income) Regulations 2002, Reg. 4(1)(e)
The general rule is that income must include an amount equal to the sum of money for which the voucher is capable of being exchanged.
The full rules can be found via the fifth row in the table in Employment Income Manual para EIM16140.