@babe I agree with @alex, your phone number is your personal data that Sainsbury is handling without your consent. I don't know what the situation is in the UK, but sending an e-mail mentioning "GDPR" usually does the trick on the continent.
@babe If they really don't verify if a given phone number opts-in to their spam you most definitely shouldn't write a script that just sends in all possible phone numbers and then you absolutely should not buy a bag of popcorn to watch how they handle the PR fallout when all of the country is bombarded with their shit, while they spend a fortune on carrier costs.