Just a quick note that of course the early war 'heavys' were initially equipped with ventral turret, later removed in most cases as they were clumsy and regarded as not much use - the Wellington and Whitley had turrets - the hole for the removed turret on the Whitley was used as the dispatch hole when they were used to trial paratroop ops.
The Hampden had a flexibly-mounded underside rear gun position as well of course, but throwing brinks was probably as effective as using the Vickers K from there.