This design has been traduced because of misuse of the word "fighter". RAF's 1936 name-change, Air Defence of G.B to Fighter Command was a budget-jerking wheeze: politicos wanted to concentrate spend on bombers. RAF/1930s bought no "fighter" because UK was to rely on the Maginot Line: the Abbeville Boys were French. Luftwaffe Army-support types - He.111/Do.17, transport-derived -would trundle beyond escort range towards Expeditionary RAF bases, to be taken out below/abeam/astern after visual intercept. No head-on, so no forward fire; no manoeuvre/second pass, so no superior energy. Phalanx (per AG) if the incomers had been early detected, or solo-lunge by a loitering patrol pair: the Task is less to destroy, more to disrupt, break the formation (only the leader could navigate), dump the load. No GCI, no nightwork. No type slowed by turret+its fingers and eyes could take on a nimble dasher: but those would be immured beyond the Rhine: Defiant would not meet one. Hurricane, Spitfire, Whirlwind (A to Flitzers Q: these were the no-turret interceptors), Bf.109, Bf.110 were similarly intended to be bomber-destroyers (Zerstorer). Defence Policy has failed if we are within range of no-endurance, point-defence sprinters. It did; we were.
The "even more melancholy story" (M.M.Postan, WW2 Official History, Design & Development of Weapons, HMSO 1964,P.135) of RN turret-fighter, and then of turret-less multi-role Skua, resulted from Admiralty perception, when extracting 1936 funds for 6 large Illustrii, that land-based Air would offer little threat to the Fleet on the High Seas - one Spec was for a 3-seat Spotter-Fighter! Deck armour was anti-plunging big-gunfire, not piffling 250lb. squibs making holes in the water.
Air Ministry/then Ministry of Aircraft Production "owned" Design Rights in everything we paid for; design parents had no right to production. MAP assigned work having regard to, for example, ease of moving material/engines around under bombing, no motorways, no street lights. So, Roc, Defiant and their BPA-licenced French turret, were all built snug in Wolverhampton; Roc-less Blackburn was put to assembling Swordfish in Barton, Pegasus from Accrington.