The BBC has an interesting article for the season of mists and mellow fruitfulness, on just when you can and can't take that fruitfulness from public and other people's property. Basically, if it's not for commercial gain then you're mostly good to go, with a few caveats such as local bye-laws. Actually the article seems to contradict itself by stating that it's technically an offence to take fruit from a neighbour's tree that hangs outside their property. So confusion still reigns!

Note that the picture is purely for illustration – I have no idea if these fungi are edible or not!