Churchill was seen as a naysayer and not particularly popular, until he was proved right. I think for a POD he would have to be proven right before the war, so perhaps after the Germans take the rest of Chzechslovakia in March 1939 the British Parliament sees that he is right and make him PM in mid '39.
I think his main contribution in coming to power would be his energy level, his penchant for saying 'Action this day!!!'. His insistence that things get done would push the British war effort up a gear sooner that IOTL. They might not have more equipment than IOTL, but I'm certain that Britain would have more ideas and more vigour in carrying them out. They might prod France into expanding the 'Saar Offensive' in 1939, they might do much better in Norway, and in the battle for France as a result of this vigour.