Booths is definitely posher than Waitrose.
As well as poshness, you could probably draw a graph of food quality and variety (because that's what actually matters) vs price.
Morrisons, Aldi and Lidl would be above the line, ie the food is better than the price would suggest, but Tesco and Co-op would be below the line, as in it's expensive for (often) crap quality. Agree about Morrisons being the best for fish, and even more so now that they've bought a Cornish supplier that is facing Brexit related struggles that they probably voted for. They had razor clams and octopus in last time I was there.
My personal bugbear with the Co-op is that, while they shout about all their 'fair trade, local, animal welfare' credentials, our local one only sells crappy water filled imported bacon, which puts a downer on the weekend 'stagger to the nearest shop for an impromptu fry up' thought.
Iceland is a curious mix of very cheap and quite expensive, but sell an awful lot more than frozen pizza. Also has a good selection of frozen fish.
Best thing to do is mix it up, vary where you go and cherry pick what you like/what you can afford from where. I shop at just about every shop that's been mentioned, and stock up every few weeks, rather than buy the same things from the same shop each week.