there are three possible turret armaments:
single Vickers K
twin Vickers K
twin Browning
1C decided to choose the weakest one. IIRC at least the twin Vickers K was in use during summer 1940.
Chinturret, actually there were also two, the first one with a single gun the second with a twin gun (i have put my books in storage in the moment
). Bit IIRC at least the single mount was also in use during 1940.
But anyway, in you get catched in a Bomber by a fighter, you are 99,9% lost. So good cant be the defence weapons