I've done a small amount of research into the BEF, and the major factors I could find are to do with their wasting of the 'phony war' period whilst the Germans were perfecting Blitzkrieg in Poland. Are there any other major factors other than this?
There isn't any real major factor that has a clear cut meaning, rather we do know that Poland was unprepared in both resources and man power. On the other hand, the German war machine had been working for years now. This was Germanies advantage, there tactics along with prepared wartime production abd suprise.