The one point I would make is that it appears (certainly before Sept 1939) that even the most slight incident caused an aircraft to be classified as DBR and then (maybe) shipped off for Instructional Purposes.. Such as this incident.

11-08-1939 L1475 64 Squadron: tyre burst while taxying at Duxford and aircraft tipped up: DBR

There is no Maintenance Serial for this one (or I can’t find it) .. but there are others.. my assumption is that they needed ‘modern’ metal fuselaged monoplanes for instructional purposes asap it was easier to pack them off to a training station and get a new one from those coming off the production line than repair it….