Bigears, I'm so sorry that happened. OP if this is a wind up it's really cruel as people have shared their stories.
It's hard to believe that anyone can have such little grasp on reality.
I love shopping in Waitrose but that's not the only place for good quality food.
We spot discounts on key long life items and stock up - pasta, rice, beans, pulses, detergents, cleaning products etc.
If you buy brand names, these are the same in Asda (or wherever) as they are in Waitrose (just usually cheaper).
Iceland which I never used to be a fan of is actually great. I'm a recent convert, it has lots of good quality veg. A freezer will really help you store food for weeks that end up being leaner due to bills / petrol whatever.
Why can't you see that you can't expect someone else to fund your lifestyle at the same level? That's not in any way unreasonable of your DM.
Try writing down what food you prepare and cost it up at different places. Look at what items you'd be prepared to lower yourself
and go to discount supermarkets (as everyone has said, a carrot is a carrot). If there's a few items you'd prefer to get where you can be sure of the provenance, go to a farmers market. Or stop eating meat. You don't need it.
If this is real, I hope that you will look back and feel embarrassed at this. Because the alternative is you don't adjust as you can't possibility lower your expectations and that could have dire consequences for your family.