This is going to be important. The holding of Calais for an extra three days has basically allowed the BEF units to maintain far more unit cohesion than OTL. This is going to have an effect on morale back home, as the people back home will be seeing the troops get off the trains with their personal weapons intact.
Sure, the heavy weapons for the most part had to be left behind, but the civilian population didn't see that sort of thing all that often anyway. What they'll see is that the Army getting off the trains with their SMLE rifles and Bren light machineguns, evacuated from the Continent still ready to defend their homes.
The stories and propaganda, with the display of captured German tanks and other equipment will be pushed hard to establish that the narrative that the BEF was let down by the collapse of the Belgians and the French. The stories about Arras, the fighting around St Omer and the Siege of Calais, where the 30th Infantry Brigade and the 8th RTR fought a German Panzer Division to a standstill and fighting until they ran out of fuel and ammunition will be used to shore up morale back home in way that wasn't possible in OTL.
I think the attitude on the home front will be less invasion panic and more, Britain alone against the Continent. Again.