Any ceasefire agreement in 1940 would have included the provision found in the non-aggression pacts signed by the Germans at this time prohibiting the signatories from sheltering organizations hostile to the other party, or from broadcasting hostile propaganda. This would have precluded the governments in exile from remaining in London, or of continuing to use the BBC world service to broadcast to their occupied countries. Individuals could have remained in England, but not the organisations.
What the various governments in exile chose to do then would depend on their individual circumstances. The Dutch would be in the best position; their government and monarchy were free and united, they controlled their entire overseas empire (a source of significant wealth), and they were recognized internationally (i.e. in Washington) as the legitimate government of the Netherlands. They might choose to relocate to Batavia (now Jakarta), capital of the Netherlands East Indies. Alternatively they may try moving to their embassy in Washington, but with an election looming the Roosevelt administration would have been reluctant to host a government lobbying for their involvement in an already lost war, and a colonial power to boot.
For the Belgians things would be significantly worse. When the government had fled to London, the king had chosen to remain in Belgium. This had led to a rift, not just between king and government, but within the ranks of the exiled government. Disunited and with a monarch that accepted the new order in Europe, their morale was extremely low and historically they explored the option of making their peace with Hitler and rejoining their king in occupied Belgium; with Britain out of the war the Belgian government would have almost certainly have done so. Individuals fearing German reprisals might have chosen to stay as private citizens in England, but the government as a whole would have gone back.
The Norwegian situation is somewhere between that of the Dutch and Belgians; king and government were all in exile and united in their opposition to the Germans and their hatred of the puppet regime in Oslo. They also enjoyed recognition in Washington. But while the Norwegian merchant fleet (the 4th largest in the world in 1940) had sailed to safety in Britain when the Germans invaded and remained loyal to the exiled government and king, this was hardly a durable long term power base. With the war over and the writ of Norwegian law extending only to the embassy building in Washington and a modern fleet of flying Dutchmen, captains, crews and their ships would drift home to their families.
The Danish king and government never went into exile, but the Danish king was also the king of Iceland. When Copenhagen was occupied, the government in Reykjavik had appointed a regent to exercise the constitutional powers that the king was deemed unable to freely carry out and asked for protection from the British. With peace, the Icelanders may choose to continue their relationship with the British; it would not be a situation the Germans could challenge or that Hitler would allow to be a deal breaker in a settlement with the British; he was happy for the British to secure their empire (and Iceland would have been seen as a legitimate British security concern), just as long as he had a free hand in Europe. Hitler would definitely prefer an Iceland occupied by Britain to one potentially occupied by the Americans.
For the Poles things would be particularly grim; making what peace they could with the Germans would simply not be an option. Their only hope would be refuge with the large Polish Diaspora in America. But recognition in isolationist America, particularly in an election year, would be very unlikely and support would be seen as impossible and pointless even if it weren’t. The Poles would also face attacks from German propaganda sources in America (never very effective), but with the Ribbentrop-Molotov Pact still in place, also from socialist and trade union movements in America (which could be very effective indeed). The Polish exile government would be called fascists and the war ‘far from being a war in defence of democracy, is in fact a war against National Socialism’ (Stalin’s words in 1940, accepted throughout the world by Comintern affiliated socialist parties and trade unions). It takes true talent to condemn fascism and defend National Socialism at the same time. Stalin’s hatred of the Poles exceeded even that of Hitler’s; in 1939 Hitler had initially intended to leave a rump Polish state as a ‘protectorate’ of the Reich, obedient to Germany but with day to day management in the hands of Polish Quislings, but Stalin had insisted in their pact that nothing of Poland must remain.
De Gaulle’s situation is probably the worst; he’d received no recognition outside the British government (the Americans continued to recognize Petain’s regime for a full year after they entered the war) and he and his followers had been condemned as traitors in France. Nor had he been particularly successful gaining support from his fellow exiles; of the more than 100,000 French servicemen that found themselves in England when France surrendered, the vast majority chose to either be repatriated to France or be interned for the duration of the war. Even many in the British establishment regarded him as a traitor to France. The British would never have handed him over to Vichy or the Germans, but they would not have allowed him to remain in England either. Most probably he would have followed his initial inclination and gone into exile in Quebec and Free France would have dissolved.