I think a population of approximately 8 million is more than big enough to justify it. They had a big enough population IOTL to build the Sentinel.
A tank which only got produced in double figures. And yes, they didn't have the industrial capacity do really build tanks on top of all their other requirements. Also, their population at the time was only around 7.1 million.
They managed to set up the Twin Wasp aero engine factory in just over a year which is arguably more difficult.
The manpower required for that and all the other stuff they're building kind of makes building a tank factory
as well, kind of a stretch.
OTL Calais was taken by Guderian's Panzer Corps after a short hard fight. This force had been going hard for two weeks and was at the end of its physical and material ability to continue the fight. These panzer divisions needed rest, resupply and servicing. There was sound reason to stop the panzers outside Dunkirk.
Here the ports garrison just got a lot tougher than the OTL vickers light tanks and the odd cruiser tank sent OTL. In street fighting my money's on the British infantry tanks causing lots of panzer and infantry casualties. Perhaps enough to force the Germans to withdraw a short distance, invest the port and wait for the main force of infantry and heavy artillery to arrive. This sounds good and takes a lot of the pressure of the Dunkirk perimeter during the early stages of the evacuation. however Calais as a port for a major evacuation isn't going to be in fit state after being fought over and the getting the full attention of the Luftwaffe. But every bomb dropped and shell thrown at Calais is one less at Dunkirk.
Holding Calais means allowing the use of Route Z for evacuations, which is shorter, and thus both quicker,
and easier to defend, so fewer bombs will be dropped regardless, plus you might be able to get some equipment out, if only the stuff that can be man-packed.
Though OTL, the US ignored Tigers in Tunisia and Panthers in Italy and kept the same 75mm for the invasion of France, sidelined the 76mm armed version of the M4A3 till Cobra.
Germans will still get the Tiger at roughly the same time and quantity, might see something like the Panther, like the various V30 designs sooner, from what goes on in North Africa
They figured those would only get produced in lower numbers though, which proved not to be true (for the Panther at least). If the Germans see the Valiant, I tihnk it's quite reasonable for them to realise that these are the new British mediums, and are going to be the main front-line vehicles, which is quite a different thing to consider.