No Churchill doesn't automatically mean the UK surrenders. Chamberlain declared war when Germany invaded Poland. In may 1940 Churchill became PM because parliament wanted someone who would fight on.
No FDR doesn't automatically mean a more isolationist US. After the invasion of Poland and the fall of France, the US voters were very worried about nazi-Germany and supported the support of the UK, even if it would risk war. Nazi-Germany ruling Europe is not good for the US. I'm very doubtful about the chances of an isolationist winning the 1940 elections.
So no, the nazi's wouldn't automatically win if these two died in the 1930s. It may improve their chances.
No FDR doesn't automatically mean a more isolationist US. After the invasion of Poland and the fall of France, the US voters were very worried about nazi-Germany and supported the support of the UK, even if it would risk war. Nazi-Germany ruling Europe is not good for the US. I'm very doubtful about the chances of an isolationist winning the 1940 elections.
So no, the nazi's wouldn't automatically win if these two died in the 1930s. It may improve their chances.