Roger
No guarantees on anything in life, never mind planning, but if you want something viable, suggest you look at least above 5 ha (12.5 acres?) as it has permitted development rights for agricultural premises. At least it starts you with an opening for a business, more than bare land can. Also look for land which is less scenic, not SSSI etc. The fewer 'brick walls' you have the better. I really think a property with an aoc can be a more viable route (just needs one person in'agriculture', and anyway many people try to get round it).
One thing I would also consider, is buying a village house with a large garden and rent some land nearby for any livestock. My daughter is same age as your son and misses her schoolmates and feels left out. Probably not so bad if not an only child. My ideal would have been half the land closer to or at edge of the village. Saves fuel/milage as well. Lots of downside to the rural idyll....