Overall, this illustration captures the market positioning of supermarkets well.

Tesco/Sainsbury's — the middle ground.
Supermarket levels, superstores
Check Amazon, then google if it's cheaper elsewhere. Just verify the seller on Trustpilot.
You can register with one of the loyalty program aggregators. For example, TopCashback or Complete Savings. Be careful: participation is often paid! However, if you regularly buy things online, these aggregators are worthwhile.
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