Well, as Doug has dropped a broad hint that it's 1943-ish, I'd guess it could be another Rembrandt meeting-related picture. Or perhaps just a bit of testing on what were by then very quiet public roads. After the basic petrol ration was abolished in mid-1942 you needed a specific authorisation to run any non-military or non-commercial vehicle on the road. The only exception was a trade plate, which could be attached to anything - road registered or not - although some sources suggest that even those had to be authorised journey-by-journey. The plate shown is a red-on-white "limited use" one (rather than a white-on-red "tax exempt") and as the letter codes correspond to the standard licensing authorities it was issued in West Sussex.
And on trade plates as well? I wonder whose? And why are only three of four wheels guarded?
At some point I hope to do a study of all the Rembrandt and other similar gatherings which took place in the war years. I haven't read all the reports of them, but I do know that on one occasion in 1944 Peter Monkhouse turned up with his Bugatti T51, having driven it all the way from Watford on trade plates. Apparently a large crowd gathered outside the hotel to watch his departure!