We tried the polytunnel route; it's a nightmare.
It would only be approved if we grew crops inside it and attempted to sell them at the gate.
We even offered to grow crops and "give" them to our landlord in exchange for rent and the planners decreed that was not a commercial operation.
So if you want to grow lettuces and shoot between the rows while they are coming up, then offer them for sale at the gate, you might get consent. If you price them all at £10 each and don't sell any, that doesn't matter; you are a commercial operation, just one that has no real idea of market pricing.