Given the industrial capacity required to manufacture the (inordinately complicated) equivalent of about half of a Bren gun and the time and skill required to attach it to a rifle not designed for it and to modify the rifle to accept it, why not just make some Brens?
Wouldn’t it have been more effective to produce even one fifth the number of Brens of whatever number of these SMLE contraptions was produced?
I also wonder whether there would have been problems in the field with the apparatus having parts exposed to dirt etc which would not have parts performing the equivalent function exposed on a Bren.
The versions which retained wood fittings forward of the breech might have been a fire risk in the hands of the gunner if used as an LMG, which might explain why some versions lose some of their wood. A related problem would be increased barrel droop from trapped heat during sustained fire with a timber encased barrel.
It seems like a Heath Robinson solution to a problem which diverted resources to no real benefit.
The only explanation that makes sense is that the Australian, New Zealand and South African versions reflect an industrial inability to produce Brens at the time, but it seems unlikely that the UK lacked that capacity.