I'm happy to agree that subscription TV services should be regarded as a 'luxury'. So, if money is tight in a household, it would make sense to cancel any subscription to Sky (etc). Indeed, there have been plenty of posts from AB members who've made such a decision to economise.
However I suspect that the reality is that those families which are genuinely the poorest have either never had subscription TV in the first place or they've already cancelled it. So telling them that they can save money by cancelling Sky (etc) won't help them.