We'd one the other week, with stall holders from Turkey and Poland. Hardly local farmers/growers gaining from their selling, or the money staying in the local community! I've lost count of the number I've walked through. Walked through because those doing the selling seem to have less idea of what they're selling than someone working in a large supermarket. "You can tell it's fresh, it's still got the earth on it!", doesn't quite cut it with me.
They are subject to fewer rules, for the same product, than any shop, hygiene standards are lower than most shops. And I've been in some dirty shops over the years. Next one you go to ask what injections the animal had, a true Farmers Market would be able to answer this. And by rights the card should be there for reference. One piece that was put in place for Farmers Markets.
If as you say you're in London, where's the nearest farm, in relation to the farmers market? I can look out the window now, and see the lights on in the cow shed of one local farm. To get an idea of the area, think "Wutherin Heights" & "Happy Valley".
16 years ago, we lost nearly ALL the butchers in the local market, due to restrictions placed upon them. Foot and Mouth. Many were also local farmers. These restrictions didn't apply to the farmers market held outside the same year. Why it should be exempt from DEFRA regulations was never answered. Why they are able to break basic health and hygiene regulations never gets answered. No inspections carried out, so no certificates can be issued.
Raise the question of the use of Paraquat, see what response you get.