The following information is the understanding of a lay person interested in mushroom collection, rather than a legal professional. As such it should not be relied upon as the definitive legal position.
The law impacts on mushroom picking in at least 5 areas:
Under English law all land is owned by someone. Unless this land is common land, open access land, or a public right of way then entering it without the owners permission is Trespass. Under these circumstances the land owner has the right to ask you to leave their land by the shortest reasonable route. However, unless you cause damage or are abusive/threatening there is nothing further the land owner can do.
Trespass is a civil offence but is rarely ever taken to court and has little chance of being successful unless damage is caused. However, if persistent Trespass can be proved, the land owner can apply for a injunction ordering you to keep off his land. A breach of such an order is 'contempt of court' - a criminal offense.
Trespass is a more serious offence under Scottish Law.
Theft Act 1968 Section 4(3) states that:
A person who picks mushrooms 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. For purposes of this subsection "mushroom" includes any fungus, and "plant" includes any shrub or tree.
This means that you can collect wild fungi on any land for your own use without it being classed as theft. This is true even if collecting whilst trespassing! A land owner who confronts you whilst trespassing on his land has no right to ask you give-up any mushrooms you have collected - they are yours.
The situation changes if you are collecting for any commercial purpose. Collecting without the land owners consent is now theft and can be prosecuted.
Although, as stated above, it is generally legal to collect wild mushrooms on any land, exceptions may be made via local bylaws. Some nature reserves and SSSIs have bylaws banning the collection of forest produce, which includes mushrooms.
There are a number of fungi which contain controlled drugs. These include the Liberty Cap and some other Psilocybes (psilocin, psilocybin ) and The Panther (bufotenine). Possession of any such mushrooms without permission, whether fresh, dried or in any form of preparation, is illegal. All of the controlled drugs found in mushrooms fall into class A (the most severe offences).
The deliberate collection of poisonous mushrooms for the purpose administration to any person (even yourself) is illegal. A number of offences may be involve here, including: