Its a bit sneaky what they've done there...
They've kind of used the law to their advantage (and slightly bent it in my opinion) - alot of people will buy the voucher and not use it so thyere a fiver up.
Where as if you were forced to buy the actual food they would be up a a fiver minus the cost of the food.
I dont know what exactly the law says but i would have thought you had to have possessed some food at some stage