This is especially egregious in the Netherlands for example, where you will regularly find moldy products shipped from across the world (Chile, New Zealand) sold at a discount (down from eye watering starting prices.)
This really doesn't pass the smell test. There are multiple very large supermarket chains here. They aren't monopsonies, by the simple fact that there are multiple. They are also large and very good at negotiating and optimising their supply chains. If any of their suppliers were to have absurdly high margins they would fuck them out of existence. Especially for commodities like farm produce which you can get almost anywhere.
The supermarkets make enormous profits, but it's nearly all due to scale. Their margins on products are in the low single digits. (In the Netherlands. The largest chain, Ahold Delhaize, does make higher margins in the US.)
The four largest chains (Albert Heijn, Jumbo, Aldi, Lidl) are all independent of each other. A bunch of smaller chains have bundled their buying power into Superunie, which in turn is part of EMD. Superunie would end up somewhere in the middle of the top 5 in the Dutch market. EMD has about 10% of the European market. There's really no monopsony here.
In this particular market: players make razor-thin margins (low single digits as mentioned before), and have regular price wars to fight over market share.
It seems 5 is enough to keep players from making excessive profits.
I do agree that most UK shops (Waitrose/ Sainsbury's like you mention) are really good compared with most stuff in Switzerland