Ebbie - I don't think you've got the food from M&S / Walmart comparison quite right.

We have Walmart over here ('cept it's called Asda) and it's nearly the opposite end of the supermarket price range from M&S. I can afford one, cos I have to, but not the other.
I don't know your more upmarket supermarkets, so can't help with the translation.

Others in UK may disagree, but in ascending order of likely prices the UK scene goes:
Netto / Lidl / Iceland
Asda
Morrisons
Tesco
Sainsburys
M&S
Waitrose
and after that you're on to London places like Fortnum & Mason or Jacksons of Piccadilly I suppose.

And for the really mean/poor amongst us, we have Costco, too - I just wish they'd sell American apple cider (=fruit juice in UK speak) in USA quantity. They're really good for bulk buy of stuff you'd pay more for elsewhere.

Paws (not a chav IMO, but with no money to spare)