As for letting Germany take the Sudetenland and annex Austria, something people seem to gloss over is that Chamberlain knew exactly what he was doing, he was buying England time. He wasn't a fool and he knew peace would not last despite the famous "peace in our time" comment. The reality was that, in 1938, England was well behind Germany in terms of ability to wage a war. The year following saw England get closer to the mark, even if they weren't up to Germany's standards. Just imagine what the Battle of Britain would have been like if the 109 had no serious competition (the Hurricane was a good plane no doubt but it wasn't on the performance level of the 109). That is just one example.
Errr...a few points.
1) The scale of power went actually against Britain, because who the hell throw away 37 well equipped and experienced divisions, including 4 armored (although their amount of actual tanks was somehow low). And 18 squadrons of airplanes (in total about 1000 planes), dated (ok, there were some really modern ones, bought from USSR, but majority was biplanes), but still flying, well armed and in some aspects outmatching the german ones. And then there is the fact that these divisions hold a ground on a complete opposite side of a Germany, entrenched in a fortification system that is not finished, but some parts of it even in their state back then easily outmatched the French / Benelux fortification. Besides, retreat plans of Czechoslovakian army were counting with taking positions and heavy defensive warfare, effectively drawing most of german forces from the "western front" (if we excule units appointed for attack on CZE, germany would have 10 infantry divisions, 1 cavalry brigade, and some rather poorly equipped reserve divisions and landwehr. There was strategicaly no better ally against Germany than Czechoslovakia (maybe Poland, but back then they were drifting towards Germany).
2) Germany was in 1938 nowhere as strong as during invasion of Poland. It lacked spare parts and ammo as its production system was expecting the war much later and pretty much focused on building machines only. There were nearly no Pz.IIIs and IVs in service, and as for Messerschmitts, only available versions were B, C and D. The gap between these and Hurris mk.I was not that big, to be honest. And their only military operation before - Anschluss, was quite fail with logistic, even without any resistance at all.
3) Some of the biggest arms factories in Europe were in Czechoslovakia. Ok, they werent technically German until 15/3/1939, but still...they could produce guns and ammo against Germany. By agreement, which led to 15/3/1939 Brits/French basically gave Germany a lot of industry and a large number of tanks and arms of good quality.
4) Munich agreement led Poles to deploy their forces at the borders of a country instead on a Vistula river, which would have been much stronger position.
5) In 1938 western border of Germany was poorly guarded. In 1939 it was not.
6) I would like to see Battle of Britain in 1938/early 1939 with no bases in France. Yes, Hamburg had airport, but you need something little bit closer.
Honestly, I cant think of even one way that Britain benefitted from Munich agreement.