I always visit Portobello and Camden Lock markets when in London.
http://shopportobello.co.uk/portobello-market-what-you-can-buy/food/
Each has food stalls. At Portobello you'll find many take away food stalls at the Golbourne end.
Loads of food stalls at Camden Lock.
I travel in November and hit each of these markets to find Xmas stocking gifts.
For Camden Lock I tube to Hampstead early, walk in the Heath then walk all the way back down to Camden Lock,. Don't care about Mother Nature. That's my traditional end of visit excursion.
Have been to Bourough but liked it more before it became " hip and happening." Now just a glorified foodie ensemble. Preferred it when it was grittier and less fashionable.
As far as great food review Timeout London online. Pretty certain you can find something that entices a visit. Same with researching London food blogs. Always something I take a gander at before I leave for London.