The Open Guide to London: the free London guide

Councils, including most London Boroughs, are required to provide plots of land for use by residents to grow food. They have a long and honourable history, particularly during wartime food shortages. For a tiny annual outlay, you can get your own little plot of land and start growing your own vegetables.

Most boroughs have long waiting lists.

The National Society of Allotment and Leisure Gardeners: http://www.nsalg.org.uk/