Global Lessons
1. Do not align yourself with people who are on the wrong side of history
Margaret Thatcher did herself no favours by praising General Pinochet, and describing Nelson Mandela as a "terrorist". Both of those incidents showed her tendency to favor authoritarian dictatorships over inclusive democrats. She was left looking like a fool when South Africa released Mandela from jail and he ultimately became President, and she would have known at the time that Pinochet was, to be blunt, an intolerant murderer.

Political and Leadership lessons
1. All leadership styles have a finite shelf life
Thatcher would have been well advised to learn from the fate of Winston Churchill, who, after leading the UK to victory in World War II, was dumped unceremoniously in November 1945, his Conservative Party soundly beaten by the Labour Party. Churchill had done a great job as a wartime leader, but the electors decided that they did not want more of the same, or a return to patrician Conservatism.
Margaret Thatcher was elected in 1979 as a crisis leader, but by 1990 the UK was no longer perceived by electors as in crisis, and she had gone from being regarded as a "strong leader" who was what the UK needed, to being regarded as a cranky bullying authoritarian who needed to STFU and go away. The electoral psychology climate had changed, her leadership style had not changed.

2. When you dispense with people, say as little as possible, and do not disrespect them publicly
Firing people is a necessary part of governance and leadership. However, there is a difference between firing people and humiliating them. Humiliation activates the worst instincts of the people being humiliated. Humiliation is what has led to more wars and terrorism than we can count. On an individual level, if you humiliate multiple people over time, it leads to your enemies banding together and plotting against you. Margaret Thatcher tended to not only dispense with people rapidly, especially if they would not shut up, she also made disparaging remarks publicly and privately about them. That eventually catches up with a leader.

3. Ideology does not trump practicality
The period from 1979 through 1985 was a time when the Conservative Party enacted a number of legal changes, and won a number of key challenges to its authority, both domestically and internationally. Although there was an ideology underpinning the policies and strategies, they were easy to sell to the electorate as necessary and desirable.
After 1985, the Conservative Party seemed to become bogged down in trying to implement policies that seemed more rooted in ideology than practicality or usefulness. The Community Charge debacle was the best example, but other policies included the attempt to weaken the power of local governments (which tended to be dominated by the Labour Party) and the attempt to force local councils to sell their housing stock to private buyers, which resulted in several corruption scandals within councils under Conservative control. To the electors, these policies seemed to be the triumph of ideology over common sense. They were unpopular, and they contributed to a slow and important erosion of support for the Conservative Party.
The loss of support was only a factor in the demise of Margaret Thatcher, what really eroded support to below critical levels was the 1988-1990 recession, which the government seemed unable to combat effectively due to disagreement over tactics. However, the focus on ideology started the erosion of support.

4. Political parties whose policies are no longer viable take a long time to sort themselves out
Margaret Thatcher and the Conservative Party were elected to office in 1979. It was 1996 before the Labour Party would win a General Election - nearly 17 years out of national power. During that period, the party went through three leaders (Michael Foot, Neil Kinnock and John Smith) before Tony Blair took charge and made the party into a unified political force once more. For most of that period, the party was riven by infighting and factionalism, as a reformist segment of the party (initially in vain) attempted to point out that the party's policies were no longer relevant or attractive to the electorate. The response from the "old guard" was that there was nothing wrong with the policies, they simply needed to be "sold" with conviction, and the electors would buy them. It took three General Election defeats, and the defection of numerous supporters to the Liberal Democrats, for the reformers to gain the ascendancy, and for the Labour Party to leave behind it's older and unattractive policies such as a committment to nationalization, unilateral nuclear disarmament etc.
The message is that Denial is a very comforting place for a failing political party to live in, and that reforming a mass market party's policies will probably take more than one electoral cycle. To back this up, the Conservative Party went through its own wilderness period after 1996, with multiple leaders (William Hague, Michael Howard) presiding over factionalism and infighting and a similar failure to win elections. Only recently did the Conservative Party win a General Election, in collaboration with the Liberal Democrats.

UK verdict
1. Taxation burden
Despite the massive cuts in direct taxation, the overall tax burden in the UK was almost unchanged at the end of the Thatcher era. Quite simply, taxes on income are noticed easily (and resented) by electorates, indirect taxes are much less noticed. The Conservative government either realized this and were being cynical (as is often the case with politicians) or they did not pay as much attention to indirect taxation. Either way, the much-ballyhooed attempt to reduce the overall levels of taxation in the UK was not a long-term success, and with recent tax increases, there has been an increase in tax avoidance once more, as wealthier people rebel against higher marginal rates on higher levels of income.

2. The Thatcher Funeral
As I write this, there is a lot of genuine anger in the UK at the decision to give Margaret Thatcher what amounts to a State Funeral in all but name. (The main reason that it is not a State Funeral is that the latter ceremony does require an Act of Parliament). Over 20 years after her departure from active politics, Margaret Thatcher remains a deeply polarizing figure in UK life. However, her impact on the politics and history of the UK, for better or for worse, outranks the impact of any political figure since the era of Winston Churchill.