Assuming the east and west have fallen Britain might not have a choice. Once its continental allies have fallen there's no chance for the British to win the war. Simply put, a ceasefire would be inevitable by that point.If Germany is smart, peace with Britain should be possible. Don't go overboard annexing parts of France, get out of Belgium, don't demand anything from Britain (like repartations or British colonies) and it shpuld be possible. Basicly I would say that Belgium is the price for peace with Britain. They officialy went to war over Belgium, so leave Belgium and Britain can even try to spindoctor the war into a win.