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Perhaps it's because of the location that it's a bit difficult. I have some time to get the delivery organized, so I'll plug away.
As for Spanish shops, they're fine as far as they go, which isn't very far. Mercadona has 35% of the market & is clean & respectable, but has a limited range like Aldi & Lidl. (Aldi, incidentally, is THE place to go for superb value Spanish wines.) We are just inside Spain on the Mediterranean coast and quite a long way from any large towns apart from Perpignan in France, where we go roughly monthly. (Except during lockdown!)
Many UK expats will go to some lengths to get hold of Shredded Wheat, curry sauces, decent tinned red salmon & corned beef, marmalade, piccalilli & so on. Life doesn't stop without these things but they significantly improve the quality of life. As an example, both piccalilli and corned beef can be bought in France, but it's junk compared with what's normal in the UK. Shredded Wheat is simply not available.
Thanks for all your very valuable input.