You won't receive change from any voucher.
However - if you (or a friend) have a loyalty card for the store - tell them to put the change onto that. They can't give you cash, but they can give you credit.
If you don't have a store card (do JJB offer one anyway?), tell them you want other vouchers by way of change. You can volunteer to pay a little money towards this - since otherwise it may mean they keep some money that is rightly yours.
Don't let the floor staff intimidate you. If necessary ask for the store manager and tell him/her you want to make the most efficient use of your voucher and you expect their help.
