Put simply, all plants, animals and fungi on private land belong to the landowner. However, because most Britons distrust and ignore mushrooms, owners are often happy to allow picking, so always ask first. If you don’t, in theory you can be sued for damage to property or for the value of any harvested mushrooms.
Given there is no scientific evidence collecting fungi damages the mycelium and the general lack of value put on wild mushrooms, prosecutions are unlikely in practice (and would also probably fail). This is particularly true since a 2006 case brought by the Forestry Commission against a commercial mushroom collector in the New Forest where the court found in favour of the picker exercising her ‘traditional rights’.
More important to most people, however, is a little known but important exemption to the general rules of property. The law recognises a malevolent landowner might try to block footpaths by prosecuting walkers who snack on blackberries or rub dock leaves on nettle stings. As a result:
Under common law it is not an offence to pick the “Four F’s”; fruit, foliage, fungi or flowers which are growing wild if they are for personal use and not for sale. This provision does not apply if the species in question is specially protected, say by listing in Schedule 8 of the Wildlife and Countryside Act. This means that anyone can pick blackberries, take ivy and holly for Christmas, gather sloes and pick mushrooms for themselves. However, this right can only be exercised where there is a legal right of access i.e. alongside public footpath or in a public place.
Countryside Council for Wales, 2003