For UK orders, you have the option to apply a discount/coupon code at checkout. Once you have entered your delivery details, you will be prompted to enter card details. Beneath the Order Summary there is a drop down option where you can add your code, titled "Got a discount code?".
For international orders, the discount/coupon code must be applied in the basket, before checkout. To access your basket, click the shopping bag symbol in the top right and click "View Basket". Beneath the Order Summary there is a drop down option where you can add your code, titled "Got a discount code?".