Impossible
Even if Germany was able to maintain a beachhead I England, which is impossible anyway as they would be quickly cut off and surrounded by the larger and more advanced royal navy, they would reach a stalemate in the Southern counties and would have to deal with guerrilla warfare Ending their possible expansion.
As with the Soviets it is impossible for Germany to properly defeat them, even if they concentrated their forces to take the Caucasus oil regions they wouldn't have the industrial capacity nor manpower to push deep into Russia.
Flagmaker is my go to program for making flags:
https://www.reddit.com/r/vexillology/comments/2ei5wf/flagmaker_17_flagmaker_jr/
They were judging how large the war would become, as well as waiting for the right reason to declare war.
It would have been impossible for Britain not to intervene, the whole reason for the war was the massive build up of military strength between the European powers (primary Britain and Germany). Conflict was inevitable, besides Britain would have intervened to support France and Russia irrespective of Belgium's invasion.
I live in Oxford, UK