ANDREW MACASKILL MARGARET Thatcher has not been in power since 1990. In fact, she's been dead since 2013. But the former Conservative prime minister has been a regular presence in this week's contest to replace Boris Johnson as Britain's next leader. Thatcher, the most polarising prime minister in modern British history, was reviled and revered in equal measure as she crushed the unions and privatised large swathes of industry. Tough and outspoken, she led the Conservatives to three election victories, governing from 1979 to 1990, the longest continuous period in office by a British premier in over 150 years. The…