REUTERSLONDON (Reuters) - Former British Prime Minister Margaret Thatcher is the most influential woman of the past 200 years, according to a survey of Britons published on Tuesday which showed men place a higher value on political influence than women do. Thatcher, who was Britain's first - and, so far, only - female prime minister, led her Conservative party to three election victories, governing from 1979 to 1990, the longest continuous term in office for a British premier in over 150 years.