I didn't say that. I think your recollection is wrong. She was very pro the single european act, but anti Maastricht. I suspect there's nothing between us, but, here you go:-
One of the more bitter aspects of the Tory infighting over the Maastricht Treaty revolved around Margaret Thatcher-practising her role as a backseat driver-coming out against Maastricht and calling for a referendum on the treaty, which a lot of people figured was because she thought it would lose. John Major and his supporters, and even a few rebels, thought that this was her getting back at him for perceived disloyalty over her removal from power and that she would have backed Maastricht like she backed the Single European Act. Thatcher, and most Maastricht rebels, insist that this was one treaty too far and that Major was a wet, pro-European, whose opt-outs didn't mean much in reality.
This often gets mixed in with debates over Heseltine's leadership challenge and Thatcher's toppling from power, but I think it's a question that doesn't get asked enough or-when it asked-the answer is stuck in re-fighting old wars. On the one hand, Thatcher was seen as having taken an anti-European turn after Bruges and someone who said "No, no, no" to a lot of what Maastricht proposed might not have gone for it. But, Bruges tends to be overstated as some kind of conversion or descent into madness. A lot of the speech is actually supportive of the project and Thatcher comes out more from retrenchment than retreat.
Also, the question doesn't get asked since it's rightly believed that Thatcher would have lost the next election. For the sake of debate, we'll presume that there's no Poll Tax or the economic troubles of '89-91 are butterflyed forward and so Thatcher has enough stable ground to fend off Meyer's challenge, Heseltine is deterred, and let's add giving Howe a cold so his speech is shorter/less effective.
Would Thatcher have signed Maastricht?
The first thing to consider are the opt-outs from the Euro and Social Chapter, along with the principle of subsidiarity. The element of personal diplomacy does matter as I doubt Thatcher's stance on German re-unification would have won her friends in Berlin and she already felt that the other leaders had broken trust over the SEA. If she doesn't get the opt-outs Major got IOTL, I can't see her agreeing to sign the treaty, but even if she did, would it have been enough?
There's also Black Wednesday and outside factors including the Danish and French referendums. Considering Thatcher was dragged kicking and screaming into the former, an earlier exit from the ERM isn't unlikely with even more complaints about Germany than OTL. Those events gave the Maastricht rebels a boost and having a Prime Minister feeling like she was right and her Cabinet was wrong isn't going to make life easy for Clarke, Hurd, etc who'd be pushing hard for Maastricht. The party's also going to be more Europsceptic at the backbench level.
Whether she agrees to sign or not, there's also the question of rebellion. If she agrees to sign, there's no spiritual leader for the rebels to go on or a belief that their leader got the job through dodgy methods, but there'll still be rebels to a government with a much smaller majority than in 1986. It'd pass, but the mythology of the Eurosceptic Right would be very different. If she doesn't sign, there's going to be Cabinet resignations and maybe even defections to the LibDems. I doubt she'd go for a referendum on the treaty while she was Prime Minister; not just because it'd be widely seen as a wrecking-tactic.
I personally think Thatcher would have been open to the treaty, but under conditions that wouldn't have been accepted and probably leading to a crisis as one member-state refuses to sign.
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