Whilst being genetically an AVRO man I’ll gladly defend the Halifax. It had a large and unrestricted enough bomb bay to allow it to carry the large and bulky parchute load that was the Jeep (car 5 cwt 4×4) and six pounder anti tank gun. This at least was semi-recessed as when carried on the Hastings the same load was truely external.
Lets not forget that the specifications for British bombers insistined on the carrige of torpedos and 2000 lb bombs this lent itself to compartmented bomb bays and bomb cells in the wings. When the airborne forces were created (by Prime Ministerial decree) the only aircarft available were redundant bombers designed to these specifications. The holes conveinently provided in the structure for the ventral turret was used for parachute exits and the supply containers were sized to fit the bomb bays / bomb cells.
The Hengist and Horsa gliders were originally designed as paratroop dropping aircraft and as such both were fitted with bomb cells to allow the dropping of the supplies containers.