I would offer the opinion that it doesn't really make much difference: I would say that what really matters is whether any development is proportionate/pertinent to the land and intended usage/operations and to the environmental setting. Also, as I recall, an agricultural PD notification will cost you £80 and a basic agri' planning app' will also cost you £80. (My 2nd planning app' cost me more than that cos I moved the dev' site, rather than argue with the planning officer, which then involved some extensive ground works increasing the development area! I'm not actually complaining cos I was of mind to relocate to the designated site idc as a phase 2 - so phase 2 was simply brought forward, albeit stretching my start-up budget.)
I would add that, from my experience, a dev' proposal related to creation of a new rural business (clearly described, but no detailed biz plan required) will likely help: it was very clear from initial contact with planning officer that I would get dev' Approval - it was just a matter of dancing around a bit.