I don't think Britain could be knocked out permentley after the Dow, though Halifax or Chaimberlain could negotiate a ceasefire to buy time. There is a possibility to keep Britain and France out of the war altogether though. After the Von Ribbintrop Moletov pact is signed Hittler does what Stalin did in real life. Sit back and let the other dammed fool attack first, then go in as a liberator. Britain and France would be faced with an interesting dilemma then, what to do? Abbandon Poland altogether or join Hittler and the Nazi's in defending Europe from the Bolshivik hoards?