Yes. The cost of the item you are purchasing doesn’t have to be the same as the balance of your card.
If you want to buy something that costs more than the balance on your gift card, all you need to do is give your retailer another payment method, like cash or debit card, to pay for the remaining cost.
Likewise, if you choose to buy something that costs less than the balance of your gift card, you can. After you make your purchase, the rest of the balance will carry over for you to use another time.
So if you buy a £50 Costa gift card, for instance, you can use it to buy lots of coffees until your balance runs out.