The 12 acres / 5 hectares only applies to agricultural holdings - not woodland. At the moment there are no regulations about minimum size of holding for woodland, other than that the planners agree the building is reasonably necessary. That is 'necessary' not that the owners of the woodland would quite like one! Obviously the larger the wood the easier it is to make the argument that it is reasonably necessary for 'forestry purposes'. Note also that there are various definitions of 'forestry' that have been applied in these contexts.
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I'd misread an article in a paper featuring Ben Law - this part of my original post has now been deleted!