This article covers various aspects of selling homegrown produce: https://www.theguardian.com/money/2013/jul/26/making-money-allotment-gardeners
It says traders regularly selling produce at markets may require registration with the council but who knows what the definition of “regularly” is?
In my experience, councils are pretty relaxed. I operated a sandwich shop some years ago and, although I had voluntarily undertaken a food safety course and had public liability insurance, no one ever came to check me out. However, I guess if there was ever to be an issue that’s when compliance would come under the microscope.
Don’t let it all put you off. Just get some advice from the council.