I found some notes and a recipe. These are reckoned edible:
Helix Nemoralis (the wood snail)
Interestingly, snail shells used to be calcined and eaten as pills. Small "dew slugs" used to be collected and after preparation, including treating with salt, added to broth to give to "backward children" to nourish them.
Here's an old English recipe for snails with notes interspersed by a modern (1950s) author:
The large edible snail, supposed relic of the Roman days, was probably imported and fattened up in England, but the largest ones in England today are as big and fat as the French snails, so anyone who enjoys Escargotis l'epicurienne can prepare them by this old English recipe.
Collect the snails into a covered tub, or onto some grass plot, and fatten them for a week, on bolted lettuce, onion, and soft oatmeal porridge. Fruit peels, savoury herbs and fresh water should be provided daily, and the snail pasture should be shaded from the direct sun (and, of course, from thrushes). The washed snails are then thrown singly into fast boiling salted water and simmered for twenty minutes, and allowed to to get cold in the broth. The shells are then emptied and the meat left in the broth while the shells are dried and polished with a scrap of fat (if you wish to make the shells really ornate they can be dabbed with gold-leaf). Arrange the shells in a line along a split cane, and set them to keep warm. In a mortar pound a small shallot, parsley, pepper, a scrap of salt ham or bacon, and a pinch of spice. When smooth, rub a gallipot with a clove of garlic, and put the seasoning into it with enough butter to melt into a smooth warm sauce. Put the snails, drained from the broth, into this strongly flavoured sauce and cook another 10 minutes till hot through. Half fill the shells with the sauce, and put the snails into them; fill up with the rest of the sauce, and sprinkle over the top some finely powdered herbs. Take any of the flavoured butter that is left and heat up with a little of the cooking broth, and send to table separately.
Lacking the silver prongs of Soho (epitomes of the two-pronged mediaeval fork), use splinters of reed or fine skewers.
I've had the store-bought snails, and have had them served in restaurants. They're quite good. Unfortunately my garden snails are much too small. I'm curious about the slugs though...
Are some snails inedible? I wouldn't have guessed that.
Probably there are not so many that are inedible as much as some may taste bad. I'm sure there are poisonous varieties but I doubt there are many species, if any, outside the tropics. Just a guess.
Could be. I'd guess edible here might mean simply "worth eating".
As far as I can gather, slugs you have to be specially careful with. Some raw foods might be good for you, but that probably doesn't include slugs - people occasionally come down with something after unwittingly eating them in salads.
You have to take out the digestive gland, and they do produce more slime than snails:
It sounds like more "in an emergency" rather than an overlooked experience for gourmands.
Well, even Frenchs do not eat slugs, AFAIK ;-)
But big, well prepared snails are nice to eat, even if the major part of the culinary pleasure comes from the garlic-butter used to cook them in the oven.
"Not all species of snail are edible, but many are. Even among the edible species, the palatability of the flesh varies from species to species. In France, the species Helix pomatia is most often eaten. The "petit-gris" Helix aspersa is also eaten, as is Helix lucorum. Several additional species are popular in Europe, see heliciculture."