Question: In which county are the headquarters of this company located that developed one of the world's first commercial jet airliners built by Alan Samuel Butler?
Sentence (from "De Havilland Comet"): Developed and manufactured by de Havilland at its Hatfield Aerodrome, Hertfordshire, United Kingdom headquarters, the Comet 1 prototype first flew on 27 July 1949.
Relevant evidence: Yes