I rarely post on here these days, but really, this thread deserves a response!

I must have been to France on my 1939 Rudge at least a dozen times, typically covering 600 miles or so in a 5-day trip, including riding at night. I do the same on my 1964 Model 18, which is a little faster, and a little more comfortable. Those trips inevitably include quite a bit of motorway/dual carriageway, and on a girder/rigid they're a literal and metaphorical PITA, a bit easier on the AJS.

I think you can cruise much faster than the 40mph you propose. Even on the Rudge, I'd keep it at 55mph or so on the motorway, and up to 65 occasionally if required. On the AJS 60-65 all the time.

Get out there and do it.