I should be able to fill in a few matters but it is rather a long time ago. I was in Sales Engineering and the HS134 was being schemed in Future Projects which was on the floor below us in the New Design Block at Hatfield.
One matter that may be relevant is that twin-engined aeroplanes were not permitted to fly more than one hour's flying time from a suitable and open airfield. As we know, the improved reliability of power units eventually allowed the permissable range to be increased and we now have twin-engined operations that would have been impossible when the HS134's market was being assessed.