What if, instead of her resigning as she does in OTL, what if she is killed by a small group of ultra-Militant left-wing trade unionists? A small band of say, four or five former miners, who set about to kill her and senior members of the government around the turn of the decade.
I could see the devastation of her assassination on the country, with the Government going into full breath and morning mode, flags at half-mast, state funeral, which would be less iffy given that she died as a serving prime minister. Also, it'd cause significant harm to the Labour movement, being seen as those who killed Thatcher, though obviously the Party would condemn the murder. Perhaps the butterflies could lead to a larger Tory majority in 1992, aided by a sympathy vote?