Really? I wonder. Northern Ireland was my country's Vietnam, and the ruling elite certainly did say that it would be intolerable to withdraw. What was intolerable for them was not 'withdrawing from the capitalist world', but the overthrow of British prestige. Northern Ireland was, Margaret Thatcher said, 'as British as Finchley' - even though a majority of its population would have preferred to be in Ireland.
The breadth of the consensus that the six counties were and always would be British was extraordinary. It ran from the far-right 'National Front' (who had fraternal links with loyalist terror groups in N.I.) right through the Tory and Labour Parties, and even took in the Communist Party of Great Britain (whose policy was to reform the N.I. state). The Guardian newspaper supported internment without trial and blamed Bernadette McAliskey and the Civil Rights protestors for the deaths on Bloody Sunday. To the left of the C.P. it was not possible to wholly endorse the British occupation, but most radicals were still more hostile to the Irish republicans than they were to the occupying army. The New Left Review consistently opposed Irish republicanism throughout the troubles. The Militant Tendency insisted it was wrong to shoot soldiers (who were just 'workers in uniform') and even the Socialist Workers Party (UK) supported the sending in of British troops ('it will give N.I. a breathing space').
I don't know enough about Vietnam, but I suspect that resistance to withdrawal was greater than you think. Certainly it seemed to traumatise the US military and political establishment to have had withdrawal forced upon them, and Foreign Affairs has not stopped talking about 'Vietnam Syndrome' ever since.