Shaggy ink caps will never appear in the shops and very rarely, if at all, on the resaurant menu, as within a few hours of picking they quickly deliquesce, that is turn into a stinking black sludge. (But I recently read of someone who succesfully dries them in a dehydrator.)
Despite this they’re quite common in summer and early autumn, appearing everywhere from disturbed ground to punching up through the pavement. And they’re delicious.
There’s been speculation that they can make some people ill if taken with alcohol, but that seems to be due to confusion with the Common Ink Cap, which looks quite different.
I’m using the phrase ‘seasonal food’ to indicate those foods which, generally, are only available in their growing season, so many are wild and all grow in the UK.