"I'd love to think that a crop spraying emphasis may aid planning permission for a grass landing strip"
You might get away with justifying a strip for the aerial application of granular fertiliser (i.e. Nitram), but would struggle to get it on the basis of spraying alone, for the reasons that Mono has stated above. But then, I gather the CAA still classify it all as 'Aerial Application' regardless of what you're plans would be.
I still believe there is terrific scope for the application of granular fertiliser by air in the UK, the only problem being that it tends to be done from March to May only, and only when the ground is too wet to get on it by more conventional means.