By Geert De Clercq and Kentaro Hamada LONDON/TOKYO (Reuters) - Toshiba Corp plans to withdraw from its lead role in projects to build nuclear plants in Britain and India, sources with direct knowledge of the matter said, marking a retreat as it wrestles with an imminent multi-billion dollar writedown. Such a move would leave Toshiba's U.S.-based Westinghouse focused on the much narrower field of nuclear reactors and services, rather than civil engineering for nuclear power plants, or their construction, the sources said.