These are shaggy ink caps, they are a species of fungus found in Britain in the autumn. They are actually edible before they start to mature too much when the fruiting bodies, the ink caps (also known as lawyers wigs) start to turn black and break down, which is where the ink bit of their name comes from. I will not post anymore images of fungi because its easy to confuse edible with poisonous. There are a few types that are unmistakable like huge parasol mushrooms with foot wide caps, also football sized and shaped fungi (forgotten the name) which when you cut them up have white mushroom flesh throughout them.
NEVER eat fungi you cannot identify. There are dangerous ones that can kill.