My local M&S is open too, and selling clothes. I don't see the harm, really. If someone is in there buying food, what harm can it do if they pick up a pack of socks or something and pay for them at the till with the foodstuffs? I take the point about small retailers having to close, but so do large shops that don't sell food.
My bedroom TV (I know, not essential) is on the blink, and I looked online for a replacement, as I am fully locked down and have been since March. There are virtually none to be had, which puzzled me at first, until the penny dropped that the factories would probably be furloughed, and the imported ones wouldn't be getting through.
If someone lived alone, and it was the only TV in the house, I would say it was fairly essential to get a new one, whether to watch the government updates, as entertainment, or just as another voice in the house. The impact of living alone, confined to the house with no TV could be quite serious - I know my mum watches far more these days than she did before lockdown. One person's essential is someone else's frivolous, which is true of a lot of items.