There are all sorts of overpriced luxuries that good sense would suggest is superfluous to what we actually need in life. Why does anyone need a Range Rover Sport, or a Rolex, or a massive house? What is the point in flying first class? Why send kinds to Eton?
The point is, the existence of luxury goods is not an insult, or not reading the room. Someone, somewhere, sees a market for it, and often, someone, somewhere, sees value in it and will buy it.
Apparently, it serves 6, so £32 per serving. If you made your own, a whole beef fillet would be creeping up towards £100 on its own. It is expensive, but if I liked mushrooms, which I don't, I'd be tempted as a centrepiece for a dinner party or something. Definitely not a random Tuesday night in front of the the tele though.
If you can afford it and think its a waste of money that could be better spent elsewhere, good on you. You pay your money you take your choice. If you simply can't afford it, add it to the list of premium items that you might feel separate rich people from their money that you wouldn't be interested in anyway.
But I can't see how it is any way offensive.