According to my battered copy of
Modern Small Arms (Salamander, 1978, Frederick Myatt) the SMG with the twisting forestock was the BSA Experimental 1949.
http://www.historicalfirearms.info/p...1949-following
Another candidate was the MCEM2, a very advanced design which suffered from a very high rate of fire. (Designer: Lt. Podsenkowsky)
http://world.guns.ru/smg/brit/mcem-2-e.html
These links are just the result of a quick search after referring to the book: armed with the names it may be possible to find better pictures.