I don't know all that much about the Argentina Junta, but my impression is that they invaded to try and invoke a rally-around the flag affect as the only way to stay in business. Therefore, even if Thatcher does issue an adequate warning to the Junta re the British government's intent to defend the Falklands should they be invaded, might the Junta just risk it anyway as a last throw of the dice to remain in power, perhaps hoping the Americans might attempt to moderate?
In any case, minus the Falklands, I still see Thatcher winning an increased majority at the poles come 1983. I'm no fan of her politics, but I can't see why the other ingredients that produced OTL's landslide (recovering economy-at least in some areas, Labour in-fighting and Foot's unpopularity as a leader) wouldn't exist ITTL. I think this could be enough to produce a 1959 Macmillan-esc majority for the conservatives-not the same result as OTL by any means, but more than workable-and a clear rebuke for the Labour manifesto. Besides, if Mrs T thinks she's not assured of Victory in 1983, she's got enough of a majority in the commons to be able to hold out until 1984 if needs be.
Assuming this result is the case, I wonder what this does to Labour post Foot's resignation, especially if the SDP hold onto enough of their boost to be able to directly take seats from Labour at the election instead of splitting the vote (something I believe is eminently feasible in this scenario). If Kinnock still emerges as leader (something I'm not too sure of here given the increased options open to Labour ITTL), could he have an easier time dealing with the militant left (given the presumably increased cohort of SDPers as a threat)? Or does he have a more difficult time of it, considering the Labour rebuke isn't as severe as OTL?
In any case, despite her increased majority, I see Thatcher's position being weaker ITTL than after 1983 in reality. Not nearly enough to put her position in immediate jeopardy, but (like post 1959 Macmillan in some ways), she'd be far more vulnerable to leadership speculation (or an outright challenge) a few years into the parliament. Given an increased majority would still be seen by Thatcher and her closest allies as a positive endorsement of her government, I still see her pushing harder for some of her more... controversial policies-and running into opposition with members of her own party-her weaker position would also mean she'd be much less able to weather any crisis that comes her way. For that reason, I put odds of her surviving to fight the following election at around 50/50 at best in this scenario. Also, there's a chance of a Labour government prior to 1990 if they get their act together here, particularly if Thatcher outstays her more limited welcome. If not, they're almost assured of victory by 1992 in my view.