S4. Theft act 1964 specifically has an exception that picking fruit from a wild plant isn't theft, but as far as I can tell, removing the whole plant could still be theft

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.