This is exactly how I’m approaching the same problem, to weight-weenie the chain set on my ‘52 Rudge. Same Raleigh proprietary BB threading. I took the old cottered spindle and measured the “B” distance, which is 55 mm. So any other steel spindle with B=55 mm and shaft diameter not too large for the British cups should fit the frame. Right now we’re not taking about other compatibility points. I’ve looked at three sets of cups for these frames, I’ve found the original races to have pretty smooth bearing tracks, certainly re-usable. Same for the cups on a 1970 Peugeot UO-8.

If you’re looking to minimize Q, nail the chainline, and not have any bolt (think Specialitee TA, Rene Herse or Stronglight 49d) or crank arm interference with the frame or the BB cups, you need to meet these requirements AND find and satisfy the constraints on the two spindle stub lengths. This is not always a problem that can be solved. One of the issues is that the English BB shells are 70 mm wide, not the 68 mm you find on the more modern Standard British stuff.

The Raleigh system (cups, spindles, cotter-pinned arm attachments, and BB shells fit together and still (!) work quite well! Too bad they weigh so darned much!