VERY COOL!! I had an early '70's Jackson single, that I bought new, from J.D.Whiskers Cycles in London, for many years. Mr. Whisker sat on an old keg and his workbench consisted of an old wooden spool for powerline cable! While I was there the coach of the British National Cycling team (or something like that), dropped in for a visit. Mr. Whisker had such a thick Cockney accent that my wife didn't understand a word he said. I remember that their short name for Campagnolo was "Campag", not Campey, like we would say! I paid $104.00 for the frame and he sold me a set of Record brakes for $50.00!
I enjoyed that bike until the early '90's, when I just could no longer ignore all of the newer technology. I was still riding sew-ups and the Clement del mundo silks were getting incredibly expensive and the newer Conti kevlar tubulars were just too hard-riding. It was a great bike, and the workmanship and paint work was impeccable.
It looks like your bike has quite a bit of redundancy in brakes. It's hard to tell, but it looks like you control both fronts from the left and both rear brakes from the right. Might be hard on the hands. I think that the canti brakes should be enough on their own, as long as you stayed out of the mountains! If it was me, I think that I would seperate them and give control of the rear disc to the stoker, along with some discussion of how hard to apply it-it is not a drag brake like the Arai, and is quite powerfull. I might even eliminate the front disc. You could compensate for its absense by going to a v-brake in the front, where most of the stopping is done anyway.
The limited gear selection will be a liability in the mountains or when riding in a group and trying to find a comfortable cadense, but you might be able to still get the old Sun Tour Ultra 6 speed with its thinner chain.
It's not a bike that I would choose to take on an extended tour, but great fun for local rides.
If you like tandeming, you might want to consider a later bike for the big rides. The later designs give a much more comfortable ride, especially for the stoker. You don't have to go with new (god forbid!), but late '80's or early '90's would put you into a modern direct lateral frame and if you get one with 145 mm spacing (or if you can spread a 140 mm frame), you can get almost the latest generation of components to work well. I went with a '93 Santana, upgraded to 9 speed, v-brakes, modern wheels, and for a really reasonable price, my wife and I are having a blast with it!
I'm with you in that I like to actually use what others would put in a museum. I noticed your retro British cars. I have a '53 MGTD that I drive all over the place!