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Your list, which Move to Cambodia considers 'the ten best supermarkets in Phnom Penh,' are described in the blog:
Phnom Penh’s supermarkets aren’t cheaper, of course, than shopping at a local market, but if you prefer your meats refrigerated, your prices fixed and access to imported products, you’ve got many options in Phnom Penh, and more are being added all of the time including organic options. This updated posts features the ten best supermarkets in Phnom Penh.
Some things are relative and items that are good value in one market may be overpriced in another, and relative in terms of opinion, but divided into your two groups: 'M&S' with a higher prices and 'ASDA' with those lower.
As you recounted in your Thai experience, you found certain shops had what you wanted at fair prices. For a sense of what those should be, the Numbeo.com site offers a useful tool that compares the average cost of items to those in other countries, e.g., enabling you to compare the costs of items in Phnom Penh to those in London.
For staples, here are average prices in Phnom Penh, followed by that amount expressed in British Pounds, with the average cost in London in parentheses.
Milk, regular, 1 gallon: 33741.42 CR = 6.71 £ (3.42 £)
Bread, 1 pound: 4977.77 CR = 0.99 £ (0.91 £)
Rice, white, 1 pound: 1609.13 CR = 0.32 £ (0.61 £)
Eggs, dozen: 6638.61 CR = 1.32 £ (2.18 £)
Local Cheese, 1 pound: 22728.95 CR = 4.52 £ (2.86 £)
Chicken Breasts, 1 pound: 8298.26 CR = 1.65 £ (3.02 £)
Beef Round, 1 pound: 20112.18 CR = 4.00 £ (3.80 £)
Apples, 1 pound: 6539.07 CR = 1.30 £ (0.88 £)
Banana, 1 pound: 1910.66 CR = 0.38 £ (0.45 £)
Oranges, 1 pound: 5731.97 CR = 1.14 £ (0.86 £)
Tomato, 1 pound: 2111.98 CR = 0.42 £ (0.87 £)
Potato, 1 pound: 3268.54 CR = 0.65 £ (0.55 £)
Onion, 1 pound: 1961.13 CR = 0.39 £ (0.49 £)
Lettuce, 1 head: 2716.23 CR = 0.54 £ (0.78 £)
Water, 1.5 liter bottle: 3268.54 CR = 0.65 £ (0.88 £)
Bottle of Wine, Mid-Range: 40324.93 CR = 8.02 £ (8.00 £)
Domestic Beer, 0.5 liter: 5482.32 CR = 1.09 £ (1.70 £)
Imported Beer, 11.2 ounce: 6890.63 CR = 1.37 £ (1.67 £)
Pack of Cigarettes, Marlboro: 6035.58 CR = 1.20 £ (9.50 £)