There are several points to disentangle here.
"Although the defeat of Thatcher by the British working class would have been a defeat for imperialism, and a similar defeat for Britain and the US now by the working class would be too, a defeat for Thatcher by Galtieri would not have been a defeat for imperialism, any more than adefeat in Iraq at the hands of the "Insurgents" would be. Or more correctly, it would not be a victory for the working class."
If we could create a mass antiwar movement here to actually influence British policy e.g. mass demonstrations linked to strikes and occupations which helped either bring the government down or get them to withdraw troops to avert such a possibility then this would both help defeat the British government and strengthen the working class here.
This is what we should be arguing for. We are not hypothetically saying we wish for the victory of one side against another but taking active steps or arguing for the labour movement to take active steps to actually contribute to the defeat of 'our' side. In 1982 it is obvious that such a movement would have been the end of Thatcher- it was the 'Falklands effect' that led to her victory in the 1983 election.
Arthur then asks what about the working class in Argentina or Iraq? It may well depend on how the victory is won- there are several possibilities of course including a victory for fascist reaction. However, should we not oppose the war here in case reactionaries are strengthened here? No- for two reasons. The first reason is that it would suggest that imperialism or British or US intervention is somehow better than local reactionary dictatorship- no, the working class in Iraq will never have power or freedom whilst the US ruling class and its local comprador bourgeois is in power. Second- we are not indifferent to the working class there. If the British working class was strong and organised enough to actually make a difference to war - lamentably far from true at the moment- then we would of course be advocating material aid to working class organisations in Iraq- resources for trade unions, material and political. Of course none of the solutions are easy- what is really needed in Iraq is for the working class to organise and discover its own power instead of being led down the abyss of Islamism which is what happens when the left refuse to fight or offer a coherent strategy.
So we argue for the military defeat of imperialism and if possible take activce steps towards it and that does not imply supporting Sadr or Galtierieri. It does mean though arguing for the working class there to overthrow those reactionaries by coming to the head of the war against imperialism so in that sense we support the military victory of Iraq or Argentina.
Secondly, the question of Israel/Palestine.
"This argument applies equally to Israel. We might not like the fact that Israel has racist Immigration Laws. We should argue against them, but the fact that it has them cannot be an argument for denying the right of self-determination for Israel, especially as almost every other nation also has racist immigration laws. That is what the SWP and others effectively do."
I don't quite understand you here. Yes we should oppose racist immigration laws - in fact all immigration controls (rights for capital to dictate immigration) status. In terms of Palestine/Israel this means we support the right of Arabs to migrate and the right of return. How is any of this opposing the right to self-detemrnation? It is a very strange argument. Imagine of a British nationalist said we should oppose immigration as its against the right of british people to self-determination. We would rightly condemn this as racist. You though support abolition of immigration controls in Israel/Palestine but raise the argument of self-determination. If another country invaded and subjugated Palestine then that may bring arguments about self-determination into play but how so now?
Thirdly, the Malvinas settlers. It wasn't about their self-determination. They were already part of wider Argentine society- if they needed health care beyond the island's facilities etc. It was about the right of Britain's ruling class to control part of the South Atlantic- it was imperialism.