A to PMN1's Q is: none. Because if it had not been Shorts that was shot-gunned into a Belfast bed, it would have been AN Other mainland firm.
1935/36: every Parent was swamped with production. Sunderland+Stirling must be farmed out to Agencies/Shadows/sub-contractors, just like every other minnow-parent. It was not for design firms to select who would be their Production Group Members. We did that, as we were paying. For the Big Bombers...
HP was fairly easy. George Nelson had been a school mate of Fred HP, who accepted that EE would revert to locos after Victory, would be no future business threat to HP, so was happy to put Hampden, then Halifax there. Vickers was utterly recalcitrant on Warwick, which they perceived themselves as owning. Air Ministry sighed and saved their strength for the later battle, where Sir R McClean had to be fired before Spitfire sub-contracting could deliver (he should have been put in the Tower). Manchester was put into MetroVick without excess fuss from HSA. So, that left Stirling to be resolved in parallel with Sunderland. Both must be moved from Rochester, in Heinkel range from Germany.
The State in 1935/36 did not own Harland: bankers did - it was near-insolvent. They had built 600 a/c in WW1 (inc. 5+3 kits HP V/1500): Churchill's The World Crisis has their “sturdy and ardent men” building warships with “extraordinary celerity”. A.M. 1935/39 would bring other marine yards into Aero, inc. Denny/Dumbarton for 240 Sunderlands: marine had both production engineering competence and distance from Heinkel. No-brainer to put Stirlings and Sunderlands there.
But if A.M. had juggled, say Manchester there, instead of next door to Avro, at Trafford Park/MetroVick...post-War the State would have had the same problem in occupying "H&W Aero"/Sydenham that we had with SB&H. Most, near all, the other wartime shadows had left Aero to try find other markets. Though we did not own H&W, we must preserve its employment base - indeed moving higher on the political agenda after Eire left the Commonwealth 18/4/49. We had been obliged to eject Oswald from his own firm and to buy it, 23/3/43; we decided in 1946 that it had no priority over all the other empty sheds, so we closed Rochester Airport Works, Medway Works 7/48. But we could not close Sydenham...and we would have found make-work if it had been Avro-and-Harland Ltd., and even if we had no equity in it (from 3/43 we owned Short's 60% of SB&H, from 6/48 82%, and from 1975, when H&W expired, 100%.)