That's an incredibly good question to which I'm not sure anyone has ever challenged before.
here's a starter for you:
The Agricultural Wages Act 1948 s.17 provides that agriculture includes: dairy farming; the production of any consumable produce which is grown for sale, or the consumption or other use for the purposes of a trade or business or of any other undertaking (whether carried out for profit or not); the use of land as grazing, meadow or pasture land, orchard or osier land or woodland or for market gardens or nursery grounds.
An agricultural worker is someone who is "employed" under a contract of service or apprenticeship to undertake agricultural work anywhere in England or Wales.
You won't get away with the Forestry angle, but you might on the Osier land bit.