Blackberries. They grow just above anywhere, especially on urban wasteland, and they produce fruit in abundance. They can be eaten raw, made into jams or jellies, or put into pies. In the North East, they tend to ripen from September onwards but in recent years, I've found them starting to ripen in August. Down in the south, they are ripe often as much as a month earlier than up north, though the gap now seems to be narrowing.
Hazel or cobb nuts. Look out for these growing along footpaths and cycle routes. At first sight, they look a little like elm trees, noth that we have many of them around now that Dutch elm disease has taken its toll. The nuts should be ready from October in the North East, earlier down south. Don't pay a fortune in the shops for hazel nuts at Xmas time. Pick them for nothing - but make sure you beat the squirrels to them!
Rose hips. Rich in vitamin C. Useful for making syrups, jams and jellies.
Wild cherries. On Sunday I picked 2kg and used them to make cherry vodka and cherry chutney (along with rhubarb and gooseberries). Frankly, I did get odd looks from people as I was picking them - I wonder just how many people realised they have a free and healthy food source virtually on their doorsteps?