Thoughts...
By definition, if someone like Healey had won the leadership, a Labour Government under his leadership would not have been "hard left" to use the expression.
Had the Falklands Campaign been a disaster with ships sunk, lives lost and the Argentineans still on the islands, it's hard to see how Thatcher could have survived but I think a more likely outcome from such an election is a Hung parliament with the Alliance parties in a strong position which would in itself mitigate against any "Hard left" Government.
I wonder what would have happened if the Westland Crisis of early 1986 had brought down the Government which isn't inconceivable. With the Tories in disarray, I could see Kinnock's Labour party doing very well but then again so would the Alliance.
A Labour victory in a 1991 or 1992 election possibly against a surviving Thatcher is much easier to imagine and while such a Government would be further to the Left than the Blair Government of OTL 1997 they would also be a long way from any Foot 1983 administration.
The only other possibility is for the 1979-81 economic and social policies to be far worse and divisive (perhaps an external factor creates a far deeper recession than in OTL) leading to a much stronger upsurge of rioting and disorder especially in the north and Scotland where manufacturing industry is ravaged and unemployment out of control.
Faced with this disorder, the Thatcher Government mobilises the Army and Emergency Powers but in a terrible incident in the Toxteth area of Liverpool, seven people are shot dead by the army for violating curfew even though they were subsequently found to be an immigrant family who had not realised a curfew was in effect.
The Home Secretary is forced to resign and a number of Conservative backbenchers, led by Ian Gilmour and James Prior, state they can no longer support the Government's economic and social policy. With their defection, Foot calls a vote of No Confidence which is won by three votes.
The General Election of October 1981 is a disaster for the Conservatives who lose 150 seats to both the Liberals and Labour. Foot finds himself leading a Labour Govenrment with a majority of 50 - Denis Healey becomes Foreign Secretary, Peter Shore the new Chancellor and Roy Hattersley the new Home Secretary.
An Emergency Budget reverses most of Howe's economic measures and imposes stringent new taxes on the wealthy and multinational companies. Foot announces the renunciation of Britain's nuclear deterrent and a plan to withdraw from the EEC.
Margaret Thatcher is removed from the leadership of the beaten Conservative Party and is replaced by Peter Walker.