They could be defeated in detail, sure, but the French lines proved pretty tough in Fall Rot. If a longer Dunkirk battle allows the French armies to bounce off a German offensive rather than merely holding it for a few days before collapsing (because they're better dug in), giving way but not giving in, the next line is going to be reinforced even more, and eventually we're back to WW1-style fronts after an initial shocking reverse for the Allies.
So putting the question differently, can Gort and the BEF extend their stay in Flanders enough to allow the Weygand Line to hold? I've no idea, because I don't have a clear picture how much time it'd take for the allies to reinforce the Weygand line more than the Germans can beat through anyhow.
I think what ever might have been is a non-starter as the British had been shocked at the poor state of the French Army during the Phoney war period as the BEF was building up in France and what little faith they had left was used up during May - not helped by the criminally ineffective French high command - principly Gamelin who had already decided how the Germans were going to attack and dismissed any and all intelligence as to how they were actually attacking until it was too late and then did very little to effectively counter it.
As far as I am concerned Gamelin was the Best commander Germany had during the Battle for France.
So at the end of the day - it does not matter what the BEF does so long as Gamelin is preparing for and then fighting a different battle to the one that actually took place.
My armchair strategy is to tell Belgium to let the French army in to help defend their borders with Germany or to 'sling their hook' and the same to the Netherlands and 'promising' them that French forces
would not come to their aid if they were invaded before giving the Entente permission to move in.
Send 7th Army to help the Netherlands army (As per the plan) and the 3 French armies to reinforce the Belgian army as an administrative move.
However Keep the Majority of the BEF (might not be possible politically but certainly would make sense given its mobility), the majority of the DCRs and DLMs and reserve Divisions held back - as a large reserve force.
As for British forces deploying directly from the UK - the BEF in May 1940 in France was about 500K strong while the British army was far larger - in June 1940 it was 1.6 million strong (obviously much of this was units still training and forming up in the UK) and so there was an opportunity for units to be shipped to the Netherlands - but the main German effort is going to be to knock France out of the war ASAP and so the British were right in deploying the BEF to France.
With no need to rush into Belgium after the Germans had attacked with the most mobile and best formations - the main defensive positions can be held by Cat A and B infantry Divisions - dug in in depth and having familiarised themselves with the local area etc with the mobile formations held in local reserve and strategic reserve.