Metropolitan Cattle Market
The Metropolitan Cattle Market, just off the Caledonian Road in the parish of Islington was built by the City of London Corporation and was opened in June 1855 by Prince Albert. The market was supplementary to the meat market at Smithfield and was established to remove the difficulty of managing live cattle at that latter site.

The Metropolitan Cattle Market, just off the Caledonian Road in the parish of Islington was built by the City of London Corporation and was opened in June 1855 by Prince Albert. The market was supplementary to the meat market at Smithfield and was established to remove the difficulty of managing live cattle at that latter site.
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