I saw one of these in Reading a couple of days ago. 10 speed, Simplex gears, 584-44 tyres (& obviously the right size), i.e. 650B, which I understand was traditionally used for French tandems. Everything fits a French origin.
Sold by The Tandem Centre, 281 Old Kent Road according to the head badge.
I'll pass by the spot I saw it locked up later, or tomorrow. I'll have another look if it's still there.