Factual error: The episode is in May 1940. The ship in the photo that causes the arrest of Thomas Kramer is a King George V class battleship. The lead ship of the class was commissioned 11 Dec 1940.
Factual error: In the sequence where Michael runs over the innkeeper Judd, the car which comes up behind Judd has very bright headlights with the characteristic three spokes towards the centre. At that time, all cars had to have masked headlights as could be seen elsewhere, including on the earlier shots of the green SS Jaguar outside the pub in daylight. Also, when the car lights are first switched on, it looks as if the later Hartley (louvered) masks were used rather than the early hooded type.
Factual error: Foyle's son is assigned to be a target for radar tracking, and he is assigned a Spitfire. However, the aircraft depicted is a four-bladed model, which didn't exist in 1940.
Answer: He doesn't actually wear it all the time even in summer. In some episodes in the summer he simply wears his suit with jacket.