"Query : did the Brits ever outfit any Lancs with lower gun positions??"
Yes, in the early days of its service. But the sighting was difficult, the 'bin' being aft of the wing trailing edge collected all kinds of muck and oil and often jammed as a result, and the drag slowed the aircraft something awful, so the 'lower mid turret' was deleted. The opening for the mounting was retained in Mk.I production but plated over; it was omitted altogether in later production runs. The RAF publication AP 2062 A, Vol.1, Sect.7, Chap.1 covering maintenance of the aircraft includes a drawing of the turret's mounting; the turret itself was covered by the manual supplied by the turret's manufacturer - Fraser-Nash, I think.
Cheers!