As Ian says the 1939 ISDT has a great back story, whether bikes were abandoned I can't recall but quite a few competitors rode their bikes home.
What is overlooked is the real help the riders were given by the German organisers to leave Germany before hostilities started, fuel, visa's etc.
Somewhere I have the story in an old issue of either Motorcycle or Motorcycling and I'd suggest that would be a good start for background.
Then the BSA owners club, maybe the ACU.
A surprising number of ex ISDT bikes survive, your may well still be in a collection, even a museum, Sammy Miller might help if he's in a good mood.
Have you checked DVLA records to see if that reg is still on their books?
There has been quite lot written about that 1939 event over the years.
Thanks for that mini-me, I do have a copy of that book (its at my mum's house somewhere) there is actually a picture of Les Ridgway in 'action' in there as well. Late last year I did manage to track down a December 1984 copy I think it was of Classic Motorcycle? with the Les Perks' article in, although not actually managed to read it yet.
I have checked with the DVLA online and nothing was showing up there so will give the BSA owners club a go see if that leads anywhere.
Can't help but best of luck in your quest.
The 1939 ISDT was the one where the British teams had to make a fairly hair-raising trip back across the continent to avoid being caught up in the war. I believe a number of bikes were abandoned in France and others were loaded onto vans and brought back to the UK. Some managed to ride their bikes back.
I have a copy of the Rob Carrick ISDT book in which he says: "Of the CSMA team, Whitehouse lost 10 and Ridgway retired, leaving Fred Perks running on time" on the penultimate day - after which all British competitors were instructed to abandon and return to the UK.
I'm pretty sure his machine made it back to the UK as I also have a letter from someone at BSA apologising for the failure of it during the event, will try to scan that on later on when I get home.
Stew