Brens were designed to fired from the hip. Apparently there was a shoulder strap which went over the right shoulder and the handle at the front could be cranked out to give a stabilising hold on the weapon. The shoulder strap was optional if you were a 1940s incarnation of Rambo.
There are videos on YouTube of infantry sections in WW2 with the Bren gunner with his gun slung as described. I think the practice dropped out of favour after WW2 with the increasing emphasis on firing from cover.
BREN Light Machine Gun: Technique and Doctrine - YouTube
Regards,
Charlie