New Covent Garden Market
New Covent Garden Market in Nine Elms, London, is the largest wholesale fruit, vegetable and flower market in the United Kingdom. It covers a site of 57 acres (23 ha) and is home to about 200 fruit, vegetable and flower companies. The market serves 40% of the fruit and vegetables eaten outside of the home in London, and provides ingredients to many of London's restaurants, hotels, schools, prisons, hospitals and catering businesses.

New Covent Garden Market in Nine Elms, London, is the largest wholesale fruit, vegetable and flower market in the United Kingdom. It covers a site of 57 acres (23 ha) and is home to about 200 fruit, vegetable and flower companies. The market serves 40% of the fruit and vegetables eaten outside of the home in London, and provides ingredients to many of London's restaurants, hotels, schools, prisons, hospitals and catering businesses.
Check the official site before travelling; hours and prices can change.
More markets

Borough Market
London's defining food market — go early Saturday, eat your way around the perimeter.

Columbia Road Flower Market
Sunday-only flower market in the East End — 8 a.m. for choice, 2 p.m. for prices.

Maltby Street Market
Borough's smaller, scruffier sibling — railway arches, weekend only.
Source: wiki-list