We should not forget that through most of the Thirties the Labour party did everything it could to oppose rearmament and preparedness,
Some of their leaders (like Lansbury) were pacifist. Others ,( like Nye Bevan), were pro Soviet,even when the facts of the deliberate starvation of millions in the Ukraine were widely known and feared that Britain's forces might be deployed against their communist Russian comrades.
It always struck me as one of history's ironies that they were able to blot all that out and come out on top, blasting "The guilty men of Munich".
Labour sympathisers, like Arnold Toynbee, were still trying to destroy national sovereignty for the will o' 'the wisp of "collective security" long after it was obvious that other means would decide matters. No wonder the Nazis called him "a most intelligent man".
Labour was broad church, and had many different strands of opinion. It was also undergoing traumatic reorganisation, with the disaffiliation of the ILP. Therefore, it is not always easy (or safe) to attribute a coherent policy line to "Labour" - as it hardly existed in a coherent form. And, while indeed there were individuals wholly opposed to re-armament, there were also others (like Morrison) who, since 1935, had been arguing for a better shelter policy.
That notwithstanding, the Tories were in power and had a majority sufficiently large to enable them to re-arm. The problem, therefore, was not Labour, but the Tories - and also their media friends. Don't forget that the Daily Mail, the Times and the Sunday Times strongly supported appeasement.