On the issue of the Mk.V's rear turret the Vickers Putnam states;
"At an acceptance conference held on 15 November, 1940...Among details proposed were that the rear gun turret could not be expected to be work-able under all conditions by a rear gunner. So the rear turrets were fixed and operated by an electro-magnetic firing control from the pressure cabin, a periscope being installed to provide observation of the rear hemisphere."
The Mk.VI was to have had a pressurised Frazer-Nash 70 turret with four .303in MGs but it was not available in time so the same solution was carried out, locking the standard Fraser-Nash 20A turret and firing the guns electro-magnetically from the pressure cabin and aimed with a periscope.
It's also worth noting that the original 3-man crew and 4-man crews did not provide for a gunner.