What reaction did Britain have to Germany's invasion of Poland in 1939?
This is a good question. There are several points that need to be made.
First, there was a policy of appeasement when it came to Germany. In other words, according to the Treaty of Versailles, Germany was not permitted to build their military beyond a certain point (as well as other stipulations). For example, Germany was only allowed 6 naval ships. However, Hitler, who was elected did not follow anything. The other European nations allowed Germany to break the treaty. They believed that the treaty was perhaps too harsh. This was the policy of appeasement.
Second, when Germany attacked Poland on September 1, 1939, France and England declared war on Germany two days later. So, it appeared that they acted swiftly, but they did very little in terms of helping Poland. They set up a blockage of ships to hurt Germany's economy. In retrospect, many see this as a mistake. All knew that Germany would become a menace. The policy of appeasement should be judged as a mistake.