Ok, I was engaging in a bit of their proudly proclaimed SPOV. If they hand it out they'd better be able to take it as well.

As context for the joke, the most middle-class of the national supermarket chains in the UK is Waitrose. (Middle-class as opposed to working-class; the upper class would not be seen in such places.) Number two is Sainsburys. So, some comedian remarked, what's the point of Sainsburys? To keep the riff-raff out of Waitrose.