This is, of course, a very large topic. Here are four of the best:
Fat Hen is one of the best kept secrets of the British countryside. It is a pioneer species – a plant whose seeds lurk in the soil waiting for somebody to turn it over, and then germinating to briefly dominate before the longer-term species take over. It is therefore very often found on ploughed arable land, or anywhere there has been earthworks (including gardens). In terms of eating, it is one of the very best. Use it like spinach, but it is much tastier. This is a plant that somehow manages to taste nutritious. My favourite thing to do with it is to fry with ripe tomatoes, nutmeg, salt and pepper. The seeds are also edible (this is a relative of Amaranth and Quinoa).
Fat Hen has numerous edible relatives – the Goosefoots and the Oraches – including one species that was once widely cultivated in Britain (Old King Henry). They can be hard to tell apart, and you do need to watch out for a couple of poisonous lookalikes, notably Thorn-apple (Datura stramonium).
Very common, and the shoots are a surprisingly good spring vegetable. The best parts for eating are the “fiddlehead” young leaf shoots, just as they start to get substantial. Cook them for a few minutes in butter – keep adding more butter as the shoots soak it up. Just don’t get this plant mixed up with it’s photo-toxic giant relative (Giant Hogweed) which can cause very serious burns and have disfigured people for life. Hogweed is also slightly photo-toxic, so be careful when collecting with bare hands in strong sunlight. Another member of the carrot family also has a reputation for causing burns in sunlight – Wild Parsnip (which is the ancestor of cultivated parsnip).
Broad-leaved everlasting pea
The whole plant is edible, but the best part is the the flowers. There are numerous members of the pea family growing wild in Britain, and many of them are edible in one way or another. The “peas” themselves are usually edible after cooking, but must be consumed in moderation. People who eat a lot of them can end up suffering from a disease called “Laythyrism”, involving tremors and muscle weakness. But the flowers and shoots of most of the peas, vetches and clovers are also edible. The only member of the family that is seriously poisonous is the largest – Laburnum, which contains cystine (a nicotine receptor agaonists that has been used as an aid to quit smoking, but is also potentially lethal).
This is a member of the Cabbage Family – a large family of plants, almost all of which are edible. Lady’s Smock is one of the larger members of the genus Cardamine, which also contains the very familiar garden weed Hairy Bittercress (C. hirsuta). Lady’s Smock is more aromatic than most of them, but makes a wonderful component of a mixed wild salad. The whole plant is edible – leaves, stems and flowers.