Section 89, ITEPA 2003
Luncheon vouchers of 15p per working day are not assessable to the employee.
When was the last time someone could buy lunch for 15p? World War Two? Oh, and if your boss offers you a voucher which can buy an actual sandwich, I suggest you decline. Because you are not going to remember to put it in your tax return, and then you will be breaking the law.