As promised, I am opening the forum for this subject of KEY interest! For the moment, I have received the suggestions below (thanks Isabelle), but I hope other people will contribute too.
Plenty of excellent farmers’/food markets in North London worth trying… Saturday mornings/early afternoons: West Hampstead, Parliament Hill and (just started) Hampstead Village (behind the Everyman cinema). All have stalls selling fresh fruit and vegetables (many organic) as well as red meat and poultry, fish and seafood, bread, cheese etc. For more information about farmers’ markets across London see lfm.org.uk.
Alternatively, or additionally, try one of the many organic box delivery companies. Most will deliver once a week and offer pretty much anything you want. Two of the best known are: Abel and Cole (riverford.co.uk).