There are now 21 farmers’ markets ringed around the capital. The newest being East Finchley. There is a full list here. Most are Sundays only with the exception of South Kensington on Saturdays and Imperial College on Tuesdays. Opening hours are strictly 10-2. All the produce must be raised, grown, produced, gathered, caught, or baked within 100 miles of the M25, although there are a few exceptions that allow for 150 miles. The rules are here. Farmers love an acronym so they are regulated by FARMA – Farmers Retail and Market Association (who don’t seem to have a web site at the moment). Outside of London the FRA – Farm Retail Association lists other local resources here