The four Fs: fungi, flowers, fruit and foliage
The Theft Act 1968, for England and Wales states:
A person who picks mushrooms (all fungi) growing wild on any land, or who picks flowers, fruit or foliage from a plant growing wild on any land, does not (although not in possession of the land) steal what he picks, unless he does it for reward or for sale or other commercial purpose
Where the public have right of access they may legally collect such items for personal use, although this right does not extend to CROW access land.
Commercial use is defined as the selling of flowers or products derived from fungi or fruit for profit.
A tree growing in the hedgerow is defined as growing wild but a tree growing in an orchard is not.
This right to forage does not apply where the site is a Site of Special Scientific Interest (SSSI).
In general, foraging for personal use is mostly legal and a useful way for people to get involved with their environment.