To use the GPDO permitted development rights you have to ask before building it. If the LPA is prepared to ignore that, then you're getting a good deal
There is no area threshold in the GPDO for forestry, unlike for agriculture.
There is no commercial requirement in the GPDO for forestry, unlike for agriculture. Managing a wood to supply firewood for yourself is still forestry.
Planners often confuse agricultural and forestry PDs and try to apply the extra agricultural restrictions to forestry.