"When I started foraging, I found the best thing to do was get a really good book on native wild plants. An excellent one is Roger Phillips's Wild Flowers of Britain. I'd go to a typical habitat and pick one of every plant I could find, go home and work through the book identifying them. Then I'd check a wild food book to see if it was edible, and find out whether I could eat it raw or needed to cook it.
There are plenty of patches of so-called waste ground in urban areas. All year round, you can find rosehip, cow parsley, alexanders roots, dock leaves, hawthorn, blackberries, burdock roots, elder, spring nettles, goose grass or hairy bittercress. All of those are typical, but it's easiest to start with things that are traditionally in common usage such as rosehips, blackberries and nettles. If you don't like nettles, you probably won't like any wild food. They're very versatile – you can use them for soups, sauces, jellies, syrups and sorbets.
Mulberries are very common. Come July or August they litter the ground in most city parks. You can just gorge on them. They're not a commercial fruit because they don't transport. Once you've picked them, they squish and you have to eat them there and then. Figs are also quite common in London. Last summer I found absolutely huge figs in a couple of parks. Dandelions are another classic, and Spear Thistle is good as well, with a nice, mild flavour, although it does make you fart a bit.
As for roadkill meat, when it's frosty, pheasants could be lying by the side of the road for a week or more and they'd still be fine to eat, as long as they looked plump and healthy and were just knocked down dead rather than splattered. After all, you would hang a lot of meats anyway, to tenderise them.
I'm eating lots of spring salads at the moment, and wild mushrooms in pasta and risotto. There are some good mushrooms around at the moment, such as Field Blewits and Jelly Ear mushrooms. Jelly Ear you can eat raw or cooked, Field Blewits you need to cook. You should definitely have a good fungus guide with you if you're picking mushrooms.
Also in season is wild garlic, which you can find in damp woodlands, particularly old, well-established woodlands. You can find garlic mustard and sorrel around there, too. Gorse blossom is very nice and has a coconut scent to it. And quince blossom has a lovely pink-to-red blossom, with a hint of almond in the taste.
It's useful to take a knife and a little trowel. But mostly I just use my bare hands, and a bag to put things in. Polythene bags can be quite good, particularly when it's hot. If you have a bit of water with you, you can put a bit on to the leaves, then shake it up in the bag if it's going to be a couple of hours before you get home. There's nothing worse than picking a lot of things and then getting home to find they've wilted and can't be revived.