I doubt the French could have mounted and sustained an offence if they had wanted to. They just didnt really have the logistical system for it. Much of it was still horse drawn, as was much of the artillery.
The BEF was fully motorized then, but small. And I dont think they could have drawn thier supplies from the French because the French ssytem wasnt set up for that. So, a creeping, small offensive was possibly about all the French could do, which is what they did.
It would have probably been a situation where they would have to stockpile supplies for a week or two, then attack a few miles and halt. Then the supply depots would have to be set up a little closer, more transport routes added, and a build up, then another attack. And so it would have gone. much along WWI standards, logistically speaking. And that if the Luftwaffe didnt object.
The Germans system was much better suited to offence, though much of it was also horse drawn then. The Germans were better off in keeping most of the panzers supplied, but even they had thier limitations in 1940, frontal units outrunning thier supplies. Luckily for them, the French and British were so disorganized by constant Luftwaffe attacks that they couldnt capitalize on that when it happened.
In 1939, the German army was clearly not prepared to attack France, and that is why, amongst other things, the attack was postponed several times.
If the allied had attacked in September 1939, the Germans would have been wiped out.
The problem was that, in 1939, Germany managed to produce enough tanks and planes, etc to be ready for the following year, while the French were still messing around.
Their best production rate for tanks and planes occured between April and June 1940, when it was already too late, and a lot of the new equipment (B1bis and S35 tanks, D520 fighters) could not be delivered to the units.
On 10th of May 1940, the best French fighter, the D520, the only one which could beat the Me109, with better manoeuvrability though slightly slower, had been delivered to only 1 squadron, which meant that, on the day of the German attack, the French Air Force had only 36 modern fighters in service! More squadrons were equiped in the following weeks though, but not enough to stop German air supremacy.
And yet, they managed to shot down around 1100 German planes in 50 days or so! Another key thing is that, every time D520 fighters met Me109, the French shot down more Me109 than they lost D520.
An explanation of the French lack of preparation and fighting spirit amongst the elite can be found in a book plublished recently from Annie lacroix-Riz, which gives a broader view on the decisions which led to the final catastrophe.
For example, during the late 1930's, some people, amongst the financial, political and industrial elite favoured Hitler as opposed to the Socialist goverment in place. They also hated communism so much that, again, they favoured Hitler who was seen as a rampart against the "reds".
That is why, for example, French steel and bauxite was sold to German industrial companies, while French factories did not produce modern tanks and planes, although everyone knew that war was ahead.
If I remember correctly, the last shipment of French raw materials to Germany took place in April or May 1940. Raw materials which were used to build the tanks and planes that invaded Poland, Checoslovakia, Belgium, the Netherlands, and France.
According to what some historians say, part of the French (and British) elite worked to prevent the war from happening too soon and hindered their own country's war effort to protect their interests, as Germany was, during the 1920' and 1930's a very big client, where they had made huge investments.
This was also true about Americans companies, like IBM, Ford and GM who had factories in Germany.
Even though some people refuse to believe this kind of facts, German testimonies prove that this was an embarrassing truth, as civilians pointed out that, during bombardments of Germany, the safest places were the Ford and GM factories, which were never bombed, while their city was flatenned...
Ford CEO during the war was awarded a medal in the 1930's by the German ambassador, and he also had a portrait of the fuhrer in his office... Hitler himself pointed out that he liked Mr Ford, as it was nice and easy to "do business with him"...
I think there are a lot of inconvenient truths about the attitude of Western governments towards Germany in the 1930's and early 40's, and the explanation of the absence of reaction to the invasion of Poland should not be justified merely on a too commonly and easily accepted pacifism amongst the population.