Amazon 'prime now' loophole
On Amazon Prime Now, if you add a physical Amazon gift card to your basket it counts towards the £20 minimum spend threshold. This means that if you only want one item that's less than £20 you can add the gift card to your basket, allowing you to reach minimum spend.
You then credit the gift card to your own account. This does mean you can only spend the voucher on amazon but this shouldn't be an issue if you're someone who uses amazon regularly.