What betrayal? The UK and France did go to war against Germany, but they were not in a position to invade Germany in 1939. France suffered more than 4 years of German occupation as a result of its decision to declare war on Germany, a pretty high price for honoring its obligations to Poland.
France did not honor it's obligations and if it wasn't in position to attack Germany then France shouldn't have made those guarantees (Maurice Gamelin's "bold relief offensive"). Thinking that Germany wouldn't have attacked France if war hadn't been declared is a bit naive.
