They weren't sitting on their asses. There was substantial naval combat from the outset.

The entire British-French plan on the Western front was fighting a defensive war on the ground. It's all well and good criticising 80 years later but the idea of launching an attack through the Siegfried line and abandoning the defensive positions along the french border was not seen particularly favourably (especially when most of the French army was organised around a defensive war and the french public was generally not in favour of a repeat of the bloody WW1 offensives)

Add on top of that the fact that Poland was defeated far quicker than was expected and it's understandable why Britain and France thought it best to remain defensive.