i think you should be allowed to choose what tail turret configuration before you sortie.
i found this on wiki
Armament
Defensive
While eight .303 in (7.7 mm) machine guns were the most common Lancaster armament, twin .50 in (12.7 mm) turrets were later available in both the tail and dorsal positions. A Nash & Thomson FN-64 periscope-sighted twin .303 in machine gun ventral turret was fitted to early aircraft, but fitting was soon discontinued and the turret often removed as it was hard to sight, especially at night. (Similar problems afflicted the ventral turret in the North American B-25C and other bombers).
When the threat of attacks from below began to be appreciated in the winter of 1943/1944, modifications were made, including downward observation blisters mounted behind the bomb aimer's blister[8] and official[9] and unofficial mounts for .50 in (12.7 mm) machine guns or even 20 mm cannon, firing through ventral holes of various designs. The fitting of these guns was hampered as the same ventral position was used for mounting the H2S blister.
