The British only had a small contribution to the Allied armies in the 'feld' in '39/40, so it wasn't up to them.
Britain also had a bigger navy than France, and used it to ensure a 'naval blockade' of Germany, as in WW1 - only snag with that, was the raw materials that Germany was getting from Russia.
France, at the top, saw the advantages of a 'long-war' its Air Force was still in process of rebuilding - when war for real happened - too many French aircraft were grounded for lack of essential equipment e.g. propellor! France, had placed large orders with US suppliers - many of which wouldn't be available until '41.
They expected the fighting when it did start to be more protracted, with preliminary bombardments as per WW1. They expected to be able to 'hold' a German attack, inflict costly casualties, and later counter-attack when the resources of the Allied empires had been gathered in.
With that view in mind, there was no point in antagonising the Germans, better leave that till 'we are ready'!!