Same size, but same quality? “Cheap” food does no one any good if the quality has an adverse effect on health

That's not the point, or sort of is but you're agreeing - you can't compare like for like 'I bought shampoo and a lasagne' because in almost any part of almost any country there's a huge range of possible costs for that.

I agree. A consequence of rising food prices may result in people opting for lower quality alternatives although, in the example above, this is just someone who always buys the cheapest regardless. There are people like this.

Quite a cherry pick. Waitrose is a high end supermarket where as Tesco is not. It's why you can only find them in the richest towns in the country.

Having eaten both 85p ready meals and £4+ ones, the products are clearly not the same either.

Not really. Sainsbury's also do a £4 lasagne and Tesco also have a premium version which I expect is about the same price.

I wasn't implying the versions are the same, merely highlighting that comparing the cost of random items is somewhat nuanced.