It's not well enough remembered, but the issue that brought down Mrs. Thatcher was her steadfast opposition to Britain entering a single European currency. Her finance minister, Geoffrey Howe, resigned over the issue. That resignation precipitated the crisis that led to Thatcher's own fall in November 1990.
As all the world now knows, she was right and her party opponents were wrong.
Here's Thatcher explaining her views in her own voice in a set piece speech:
And here she is humorously reaffirming them as only she could in a farewell parliamentary debate.