I think it's probably just a little to obscure here, but the first commercial airliner under Hawker Siddeley ownership appears to be : The Armstrong Whitworth Ensign

Armstrong Whitworth AW.27 Ensign I G-ADSR (16138273911) - Armstrong Whitworth Ensign - Wikipedia

Introduced (finally) to Imperial Airways in 1938. 4-engined, all-metal (unusual then), high wing, taildragger, about 40 passengers. Grossly under-engined, had a string of aerodynamic blunders, and other issues. It didn't last long. Never really got into service before WW2 started, by the end it was completely outclassed.

The corporate structure is more than a little complicated, A.W. came out of a major shipbuilder who had expanded greatly on naval ships in WW1, got into aircraft in a limited way, then various parts of the business got merged or sold off. An engine builder, Siddeley, came into the organisation, so they were Armstrong Siddeley, then some aircraft building was sold off to Vickers, who became Vickers Armstrong. So there were three separate aero companies by the 1930s with Armstrong in their name ! Hawker came along and bought the aircraft and engine building, calling the group Hawker Siddeley but still keeping all the constituents names. Eventually they sold off the engines to Bristol, so now there was Bristol Siddeley. Confused ? Eventually, in the 1950s-60s, Bristol Siddeley got merged into Rolls-Royce, Vickers Armstrong went (along with the aircraft side of Bristol) into BAC, and Armstrong Whitworth, like Avro, De Havilland, etc, started trading under the Hawker Siddeley name.

Just to give an indication of the Ensign's troubles, the first three of them were dispatched to great fanfare in early December 1938 with the grand Christmas Air Mail operation to India and Australia. All three completely broke down before they were hardly out of Europe, previous generation aircraft had to be rushed to pick things up, and the Christmas mails finally got delivered after New Year. And the Ensign, alas, went downhill from that ...