Ohh, that's a real shame mate, gutted for you.
I know what you mean about changing the tube, paint, let alone chrome. It all starts to add up and while its always doable sometimes you have that little voice asking if its really justifiable. Usually I ignore it, but it doesn't mean its not there.
I would imagine that by the time you put in a new tube and got it rechromed, maybe painted you could have already bought a 653 Bob Jackson off the shelf in one colour, new. But, that's not why we are gathering here on this forum is it? We don't work on reality and logic, we deal in old steal and its preservation
My world would maybe see me price up Bob Jackson putting in a new tube, blasting the whole frame and getting it painted black, leaving the 1/2 stays chromed.
Currently at BJ's your looking at a new downtube at £85 which is pretty good I think, but the £495 for the chrome is a real killer. + £22 quid for gear bosses, £10 for water bottle mount then £100 for 1 colour black enamel. Forget the chrome and its a cheap fix with a couple of quid to mask the rear chrome and a few more for decals it shouldn't be too bad. I would look at it as £250 or thereabouts to keep it a rideable and nicely renovated Bob Jackson as financially it would not be a starter with the chrome, though if you could afford it then go for it.
You use it, you clearly really like it, so monetary value means little in this case.
Best of luck with it whatever you do
Jamie