https://mycoupons.sainsburys.co.uk/?

They can be used for in store shopping as well as online.
Just register your nectar card(s)* at the link and when you log in you can click and upload maximum of 5 coupons at a time to any one card. Then just buy the product(s), have your card scanned at the till and a few days later the points will be validated and added to your nectar account.