Roughbert,
Actually yes, and in fact this IS agriculture as defined by the town and country planning act, in my opinion you would stand a chance based on the following provisions:
1) You will not qualify for permitted development for your barn on a unit less than 12.5 acres so you will need a rock solid business plan to convince the council you need your infrastructure/
2) If you get this permission, you are able to stay on site whilst implementing the above.
3) You would need to be running a very intensive, closed environment propagation system which runs all year and depends upon automatic crop protection systems running smoothly-heat,humidity,irrigation etc. They will say you can fully rely on automated controls, it is up to you to prove otherwise if you can nb take this element VERY seriously.
I completely agree with you such businesses have the ability to be far far more profitable than animal bases enterprises, and if you are specialising in permaculture trees and plants you are helping to create a more sustainable future for us all.
The thing that I am now realising however is when councils and government corporations speak of 'sustainability' they do not mean within the context of what's best for the masses, they mean sustainability as it relates to corporate interests, I hate to admit to myself this is true but it's the only explanation that makes any sense in light of the decisions we see being made around us.
Good luck