Just some thoughts on this.
Margaret Thatcher is somehow compelled to step down as Prime Minister in the early autumn of 1989, let's say shortly before Nigel Lawson's OTL resignation. Not killed or maimed, but out of high office. She is succeeded by Howe as Conservative leader. Is this plausible?
More questions.
How much more pro-European will Howe's government be over the period 1989-92 compared to the OTL governments of Thatcher and Major?
Is it plausible to have Lawson moved by Howe to the Foreign Office, to make room at the Treasury for Major?
Does Michael Heseltine make a return to the Government?
How will Labour respond to all of this?
And, long term, what sort of impact on the Conservative Party does no 1990 coup have? The splits are still there over Europe, but here, Thatcher's deposition hasn't introduced the real poison to the system. Thoughts?