He spoke to his suppliers and supermarkets just won't pay the higher prices. The supply is there though. He had a full range of salad products.
The UK buys a lot of produce from NL (which is a huge exporter of tomatoes and other greenhouse-grown produce), they have been selling in Europe at the higher prices that factor in higher electricity cost, UK supermarkets have stopped buying from them because it would mean having to increase prices.
And finally, UK producers chose not to produce at all this winter because they believed that UK supermarkets wouldn't offer a price that made production profitable...they were right.
This kind of thing happens because, for multiple reasons, there is an obsession with providing very low food prices. UK supermarkets achieve this (the prices for vegetables are comically low here, still...my Tesco had a sale on potatoes the other week, 20p/kg...how? How is this possible? Farmers here get absolutely abused) but the result is that sometimes they don't have anything.
It has been happening with eggs, milk, meat (most UK supermarkets just don't get quality meat anymore, even their premium ranges of top cuts of beef are shoe leather), vegetables...and, of course, all anyone can talk about is the declinist nonsense rather than look at how fucked the food supply chain is.