I think there are plenty of ways to stop the cut through traffic rhsdunlop. Maybe you are concerned about the effects of stopping it ( on other roads)? Residents of Manor Mount probably have a better environment now, as could residents of Devonshire Rd. Traffic could be routed along the main road ( south Circular) THe increase in traffic on that road may persuade less people to drive. The logical alternative of not doing so is that one day, perhaps in ten years, Devonshire Rd will be as congested as the South Circular. You are right I think, that the calming on Manor Mount makes more traffic use Devonshire, but in my opinion a commuter driving from Bromley to central london each day should not be driving along Manor Mount or Devonshire Rd, and then Dunoon Rd or Benson Rd ( and with better transport options hopefully less should be driving at all) That particular commuter will probably have a different opinion, but tey are free to voice it. There is also the point that Devonshire Rd only works as a cut through because of illegal parking on the pavement creating the room for it to be so, so unless we have an official policy of taking pavement space away from pedestrians to speed up/increase capacity for car use of residential roads, we should perhaps be doing something about it.
At the same time as trying to sort out local traffic issues it is also important to try to sort out a proper transport policy but I don't think that should be a brake on sorting out the local problem.