Margaret Thatcher was a big fan of the EU single market.
https://www.cam.ac.uk/research/news/thatcher-papers-for-1988-reveal-her-deep-enthusiasm-for-the-single-market
That would have made for some interesting conversations with the right.wing brexiteers on here!

I started reading it. While the headline seems to support your contention, the opening paragraphs make it clear that Margaret Thatcher was a fan of the EU as it then was, not the EU it would become:

Her speechwriting files for Bruges, including drafts and contributions from outsiders, are among more than 40,000 pages of Lady Thatcher’s papers for the year 1988 being opened to the public at Churchill College from Monday.
They show that rather than acting as a call-to-arms for Eurosceptics and attacking the principles behind the single market – of which Thatcher was something of a devotee – her speech was more concerned with the perceived power grab by European Commission chief Jacques Delors, and a possible move to a more ‘federal’ European ‘super-state’.
Historian Chris Collins of the Margaret Thatcher Foundation, the only person to date to have read all 40,000 pages of material being released, said: “She wanted her speech to be about direction, rather than point scoring – and she edges back from attacking the Commission, approaching it in a more intellectual style.
“I know she was uncomfortable about the venue, but we are very lucky in that few of her speeches remain in such a complete form as this.
“When you read her papers for 1988, you see her sheer level of enthusiasm for the single market. She goes up hill and down dale with deep enthusiasm because this is practical Europe, this is how it works together. The role of speechwriter Hugh Thomas – a committed Europhile – is also crucial to consider when looking at this speech from a historical perspective.”
The section in bold illustrates exactly my point.