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.