The Lady's not for turning.
First female Prime Minister of the United Kingdom, holding the post between 1979 and 1990 and thus being the longest-ruling Prime Minister of modern times. She served as a minister in the Conservative government of Edward Heath in the early 1970s, being responsible for the withdrawal of free milk for schoolchildren (hence her nickname 'Milk-snatcher Thatcher') and, more significantly but less well known, the redirection of Britain's science and technology research towards more utilitarian goals. After Heath lost the 1974 election to Labour, Thatcher unexpectedly became Tory leader; Heath never forgave her for this perceived backstab. Most commentators predicted Thatcher would be a temporary leader and soon disposed of, but with the financial crises wracking Britain in the late 1970s, at the 1979 general election Labour was thrown out and the Tories swept to power.
Thatcher's government was noted for being the first Tory government (as opposed to Heath, Macmillan, and even Churchill) to abandon the postwar socialist consensus put in place by Clement Attlee. Key services and industries were privatised, and the powerful trade unions were taken on directly, notably in the miners' strikes of the 1980s. This policy made Thatcher easily the most controversial and divisive prime minister of modern times; broadly speaking, about half the country worshipped her and the other half considered her to be the spawn of Satan. Surprisingly, this division still holds out today: even people with no other interest in politics can be expected to have a strongly pro- or anti-view of Thatcher.
It seems likely the Tories would have lost the 1983 general election had it not been for the Falklands Conflict in 1982, when Argentina invaded the Falkland Islands and British Antarctic Territories, and a British task force sent to the region retook them. This led to an explosion of national pride and increased respect for Thatcher's leadership in some quarters, which combined with far-left unelectability and civil war in the Labour Party to leave the Tories comfortably in power not only for Thatcher's entire reign, but for seven years afterwards (see: John Major).
Commonly called the Iron Lady, she was closely aligned with Ronald Reagan as President of the United States and confrontational towards European leaders such as Jacques Mitterand and Helmut Kohl, notably being against German reunification.