When M&S can sell two actually pretty decent pizzas and two sides for £10, you can see that Pizza delivery companies (in general) have pretty sizeable profit margins.
It's not like the poor delivery driver is getting paid a hefty chunk if it either. If they're salaried staff maybe they enjoy proper staff benefits and pay, but if they use Just-eat or the like then it's very cut throat - but the delivery companies themselves are enjoying a very nice cut no matter what. They're also forcing prices up as they sit back and let the money flood in.
Fast food has now become crazy expensive and out of the reach of many, so I assume that's the reason for the loss in revenue. Less customers isn't great when your overheads are the same or went up, so the only option appears to be to increase prices to cover the gap in sales.
I'd have thought lowering prices might help, but maybe there just aren't enough customers full stop as hple have less disposable income and high streets in many places are now swamped, albeit more likely with kebab shops that can also do pizzas of varying quality.
But, like M&S pizza meal deals and others, supermarkets now offer some decent pizzas and it's not difficult to make your own - although I don't think many people think to do that.