Imagine that you are an advisor to British Prime Minister, Neville Chamberlain, in the late 1930s. Chamberlain is deeply worried that Europe is on...

First off, since Chamberlain was infamously in favor of the policy known as appeasement, in which he and France allowed Hitler and the Nazi Germany to invade surrounding territories without consequence, and to break the terms of the Treaty of Versailles with impunity, the first policy suggestion I would make would be to adopt Winston Churchill's more aggressive stance toward Hitler instead.

Churchill advocated intervening to stop Nazi remilitarization and invasion of the Sudetenland, which Hitler claimed was rightfully German. I would suggest that the British Army and Navy convince France to join forces to push the Nazis back from the Sudetenland as soon as Chamberlin took office. 

Second, instead of turning a blind eye to Hitler and Mussolini's support of Franco's rebels in Spain, who were attempting to overthrow the legitimately elected government of that country, I would instead recommend that Chamberlin send military and economic aid to the Spanish government to repel the rebels, thereby preventing the Nazis from gaining an ally in Spain. Due to the fact that Britain formally recognized the left-leaning Spanish government, I would recommend that Britain do everything in its power to make sure that the Spanish government stayed in power and became a strong ally of the British and French, in order to act as a counterweight to Germany, and to give the Nazis another border that they would have to defend, should war break out.

Third, I would advise that Chamberlin give France full military assistance and even troops, if they wanted them, in order to defend the French border with Germany in the south and the north. This would make it clear to Hitler that both the Maginot Line, as well as France's northern border with Germany, could not be broken without incurring enormous casualties. 

These proactive and aggressive counter-measures might not have stopped Hitler from trying to invade France and England, but they certainly would have made any military offensive a lot harder and less likely to succeed.