Well, Stalin did see it coming, he just didn't expect it to happen *this* quickly...
As for the UK... could Germany have won after Dunkirk? Perhaps, if Hitler hadn't intervened. You see, after some lone German bomber attacked London (mistake), the Brits attacked Berlin. That led to Hitler getting angry, and ordering an all-out attack on London.
Had he not done that, the Germans might have succeeded in gaining air-superiority in the south of the UK. That would have allowed a landing on the coasts there, with U-boats, surface ships and simple mine-fields defending the flanks. They might have driven far enough inland to shatter the British defences.
The UK was *not* ready to fight at that time - they had left most of their heavy weapons back in France during the Dunkirk evacuation. Their defences were also very static, which is exactly what you shouldn't do during a blitzkrieg. Only later did they implement a system of mobile defensive army groups.
Even with most brits supporting the troops, and even with the British navy being very strong, Germany might *still* have succeeded. After all, the British fleet would have had to choose between not doing anything and being safe, or going south to attack the invasion fleet, and being open to air/naval attack. The outcome would not have been certain for either side...