Basically, if the war starts in 1939, and England is not neutral or allied to Germany, you are going have to come up with some scenario in which the UK either is occupied or a separate peace is made. The latter will not happen so long as the Nazis occupy the invasion beaches in the Low Countries. "British Belgium" is a possibility, however.
The easy option here is have Lord Halifax become PM, under which Churchill would probably be given a far reaching defense portfolio, but reaching an amiable peace with Germany before the end of May 1940 by way of the Italians and their ambassador Bastianini with whom relations were somewhat stronger)
The peace terms would probably have been fairly lighthanded on the UK, with the return of their mandates over the German colonies granted to then under Versailles and the de-militarisation of Gibraltar, Malta and the installation of a German base on Cyprus. There would of course have been an immediate general election, the first since 1935 so who knows how that would turn out. I can't see Churchill or Bevin playing ball in Halifax's coalition and there could be a re-alignment after the election, but one that would not jeopardise the peace treaty.
The US, where support for the UK had been brewing during May would probably focus more closely on the Pacific, would enter the war, but would not pay much attention to the European theatre. Cue the Soviets, who would still have been invaded in 1941; I can see them 'liberating' Europe almost single handedly.
