If Galtieri had won the Falklands war, it would have been a blow to Thatcher? Undoubtedly.
However, since 1945 the British state has survived several defeats in struggles to keep territory. Defeat in the Falklands war would not have shattered the state. At most it would have led to Thatcher losing the next general election (and even that is not certain).
A Labour, or Labour/ Liberal/ SDP coalition, government in 1983 would have allowed some respite to the British labour movement? Yes. But you have to be very optimistic about the Labour Party to suppose that it would have fundamentally changed the whole pattern of development in Britain - that the Labour, or Labour/ Liberal/ SDP government, would have been something radically different from France's SP administrations of the 1980s, or Australia's Labor governments of 1983-96.
About Argentina, we do not have to speculate about the consequences of defeat. They were highly positive. The military dictatorship fell, and remains fallen.
Best, of course, if we could have managed it, the defeat of both sides by working-class uprisings. But on what other than narrow British-nationalist criteria do the possible positive results for the British working class from Thatcher losing outweigh the positive results for the Argentine working class of Galtieri losing so much that we should have positively favoured Galtieri's victory?