LONDON/TOKYO (Reuters) - Honda will close its only British car plant in 2021 with the loss of up to 3,500 jobs, a major departure of Japanese investment announced just over a month before the United Kingdom is due to leave the European Union.The Japanese firm, which builds a tenth of Britain's 1.5 million cars at its Swindon plant, said the move was not related to Brexit and that it needed to focus its activity in regions where it expects to sell most cars, after struggling in Europe.But the..