I would imagine that it would be disastrous for the Allies. Without the troop commitments in Russia, Hitler would be able to concentrate on the West. Especially since for the later half of 1940 until the US entry into the war in late 41 early 42, I think it was pretty much just Britain against Germany. Not good odds if Germany isn't fighting on 2 fronts.
Why is it 'not good odds'? Our prospects for beating Germany on our lonesome are not big because, and here's the thing,
Britain was on about an equal footing with Germany in terms of resources although not, in the early parts of the war, military efficiency. But a) people worship the flashy Wehrmacht and forget the formidable and effective organisational infrastructure behind our victory in the BoB, or our successful innovation in the Atlantic war and b) as true Britons know, taking war too seriously is unsporting and precisely the sort of thing Germans do.

What exactly is Germany going to do to us?
To nick Faeelin's phrase, people frequently buy Britain's own romantic self-image and assume we were a Green and Pleasant Land of shopkeeper's and inept bank-clerks leading home-guard platoons, rather than a nation that was in much the same league as Germany concerning industrial productivity and had access to world-wide imperial and investment resources, and was heavily mobilised to boot.
Another element that is forgotten is that Germany was only able to mobilise so much of its manpower by relying on slaves taken from the Soviet Union to the tune of a couple of millions. Obviously Soviet neutrality means in the immediate term that the Germans have vastly more manpower at their disposal for use against the Commonwealth in the armed forces or in industry.
But over the long haul, they physically
can't mobilise as many soldiers as they did and still build them weapons. Not unless they start to treat the French and Dutch the way they treated Poles and Ukrainians, and that causes more problems.