Here you go - Theft Act 1968.
The bit you need is in Section 4 Sub-Section 3:
An Act to revise the law of England and Wales as to theft and similar or associated offences, and in connection therewith to make provision as to criminal proceedings by one party to a marriage against the other, and to make certain amendments extending beyond England and Wales in the Post...
www.legislation.gov.uk
'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.'
If you collect to sell (presumably any amount) then you've broken the law.
The trick will be proving the person intends to sell, and I imagine that will be down to the interpretation of the court and any evidence.
But commercial foraging is illegal in the UK.
P.S. Looks like they can't spell 'carcass' correctly in Sub-Section 4...