Asda vs. Costco?

I've long been a grocery delivery sort of a person. For at least 10 years now, given that I don't drive and tend to live in places with small, higher-cost mini supermarkets nearby. I've been to Costco once before around 10 years ago, and I went with someone who knew which prices were better than elsewhere or not. I work in education so I'm eligible for a membership which is £33.60 (incl. tax) for one year.

Today, a friend with a membership kindly let me tag along on a Costco trip. I took as many photos as I could of various items I frequently buy, so I could go home and compare prices.

(While I was there I also picked up some bagels, apples, and sweet peppers. Why did no-one tell me how great Costco bagels are?)

Let's look at my price comparisons... 

A couple of notes: I tried my best to compare like-for-like, including brands, on Asda's grocery delivery selection. That's because it's realistically where I'll do the bulk of my shopping outside of fresh produce. I do occasionally bike over to Aldi or Lidl for good deals but it's not always doable.

I would like to get to a point where I am spending less on groceries per quarter or year by buying in (relative) bulk, so I think the answer is to just get a membership and keep track of savings over a year to see how I do. The most important thing is probably just to bear in mind that it's not necessarily going to be cheaper than elsewhere! (See crossed out items above.)