During checkout you will be asked if you want to send the gift card coupon to yourself or to another email address. If you send to another email address you can add your own message.
You can view any coupons you have on your account page and you can continue to use a gift card coupon until the full amount has been used. i.e you have £30 gift card coupon and your basket totals £10. Your gift card coupon can be used and you will still have £20 credit left.