There is a lot of background necessary to answer this question and I'm sure there are others who have a better knowledge of how to answer it but here's an "abridged" version that I gained from Max Hastings' Inferno (which I highly recommend).

First, you have to remember that World War I was still fresh in the minds of all combatants, especially France, in which some of the most devastating battles of the war were fought. Therefore, neither Britain nor France were eager to fight another war. They did everything possible to avoid entering a war with Germany. The main reason for them being "allies" with each other and declaring war on Germany was to deter Germany from starting another war. As a result of the above scars from WWI, France and Britain were wholly unprepared to enter an armed conflict, as they lacked manpower and equipment necessary to fight a much better prepared German army.

Secondly, in the event of a war with Germany, France planned for a defensive war rather than an offensive one. I don't have the best grasp of this so I'll let others explain, but this is where you get the Maginot Line and a bunch of fortifications that France had erected pre-WWII in order to defend their borders from a German invasion. In any event, France had no real plans to attack Germany, at least until it had sufficient amounts of manpower and arms (Hastings states that the French intended to be fully mobilized by late 1942/1943).

Third, Britain and France were allies who distrusted each other and whose citizens largely detested each other. Again, I cannot adequately explain the reasons behind this, but many French did not trust or like the British and vice versa. In fact, Hastings points out that a significant portion of France's citizens and army supported Germany rather than Britain. This is evidenced by Vichy France and by the Vichy French armies and navies fighting against British and US forces later in the war. So, when Poland was invaded Britain and France did not trust each other enough to launch an organized offensive. There were several plans drawn up, but all were scrapped either by British or French refusal. This frustration later erupted when the BEF evacuated France.

I hope this at least answers some of your questions. Sorry I didn't go into much detail but if you're looking for more information I'd recommend reading Inferno or Koskodan's The Untold Story of Poland's Forces in World War II .

Related question: did France and Britain (or maybe just Britain) launch a major naval offensive against Germany during the invasion of Poland?

I seem to remember a quote from Churchill's books that went something like: The Phoney War was not phoney at all at sea. The war began immediately.