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?

Context paragraphs:
International Aero Engines: IAE International Aero Engines AG is a Zürich-registered joint venture manufacturing company founded in 1983 to develop an aircraft engine to address the 150-seat single aisle aircraft market.  The collaboration, between four of the world’s leading aero engine manufacturers, produced the V2500 – the second most successful commercial jet engine program in production today in terms of volume, and the third most successful commercial jet engine program in aviation history.

Boeing 777: The Boeing 777 is a family of long-range wide-body twin-engine jet airliners developed and manufactured by Boeing Commercial Airplanes.  It is the world's largest twinjet and has a typical seating capacity of 314 to 396 passengers, with a range of 5240 to .  Commonly referred to as the "Triple Seven", its distinguishing features include the largest-diameter turbofan engines of any aircraft, six wheels on each main landing gear, fully circular fuselage cross-section, and a blade-shaped tail cone.  Developed in consultation with eight major airlines, the 777 was designed to replace older wide-body airliners and bridge the capacity difference between Boeing's 767 and 747.  As Boeing's first fly-by-wire airliner, it has computer-mediated controls.  It was also the first commercial aircraft to be designed entirely with computer-aided design.

McDonnell Douglas MD-80: The McDonnell Douglas MD-80 is a series of twin-engine, short- to medium-range, single-aisle commercial jet airliners.  It was lengthened and updated from the DC-9.  This series can seat from 130 to 172 passengers depending on variant and seating configuration.

De Havilland DH.37: The de Havilland DH.37 was a British three-seat sporting biplane of the 1920s designed and built by de Havilland for Alan Samuel Butler.  The first —named Sylvia after Butler's sister— of the two examples built flew extensively for five years before being converted to a single-seater and having its engine upgraded to a 300 hp (224 kW) A.D.C. Nimbus .  It crashed in June 1927.

Alan Samuel Butler: Alan Butler (22 November 1898 —24 May 1987), full name Alan Samuel Butler, was (claimed his obituary in "The Times") the first private aeroplane owner-driver.  From 1923 Butler was chairman of De Havilland Aircraft Company —which he financed— until 1950, a year when De Havilland employed 20,000 people and was building Comet, the world's first commercial jet airliner.

Air Partner: Air Partner Plc is a global aviation services group.  The Group is structured into four reporting divisions: Commercial Jet air charter, Private Jet Charter, air freight and Baines Simmons.  The Commercial Jet division charters large airliners to move groups of any size.  They also provide remarketing programmes for all types of commercial and corporate aircraft.  Air Partner is headquartered alongside Gatwick airport in the United Kingdom.  Air Partner operates 24/7 year-round and has 20 offices across Europe, North America, the Middle East and Asia.  The company is publicly listed on the London Stock Exchange as LSE: [ AIR] , and is also ISO 9001:2008 compliant for commercial airline and private jet solutions worldwide.

Collapse of the World Trade Center: The Twin Towers of the World Trade Center collapsed on September 11, 2001, as a result of being struck by two jet airliners hijacked by 10 terrorists affiliated with al-Qaeda, during the September 11 attacks.  Two of the four hijacked airliners crashed into the Twin Towers, one into the North Tower (1 World Trade Center) and the other into the South Tower (2 World Trade Center).  The collapse of the Twin Towers destroyed the rest of the complex, and debris from the collapsing towers severely damaged or destroyed more than a dozen other adjacent and nearby structures.  The South Tower collapsed at 9:59 am, less than an hour after being hit by the second hijacked airliner, and at 10:28 am the North Tower collapsed.  Later that day, 7 World Trade Center collapsed at 5:21 pm from fires that had started when the North Tower collapsed.  As a result of the attacks to the towers, a total of 2,763 people died.  Of the people who died in the towers, 2,192 were civilians, 343 were firefighters, and 71 law enforcement officers. Aboard the two airplanes, 147 civilians and 10 hijackers also died.

FBA 21: The FBA 21 and 23 were small flying boat airliners built in France in the mid-1920s.  Their development was an attempt by FBA to develop a commercial version of their FBA 19 bomber which had failed to attract orders from military buyers.  Retaining the same basic design as their predecessor, the Model 21 added an enclosed cabin for four passengers.  Unfortunately for FBA, they aroused as little interest as their military counterparts, and only a handful were built in a number of slight variations, including one example of a dedicated mail plane.

Undercarriage arrangements: Below are featured the wheeled undercarriage (also called landing gear) arrangements of some modern commercial jet airliners and large military aircraft.

De Havilland Comet: The de Havilland DH 106 Comet was the world's first commercial jetliner.  Developed and manufactured by de Havilland at its Hatfield Aerodrome, Hertfordshire, United Kingdom headquarters, the Comet 1 prototype first flew on 27 July 1949.  It featured an aerodynamically clean design with four de Havilland Ghost turbojet engines buried in the wings, a pressurised fuselage, and large square windows.  For the era, it offered a relatively quiet, comfortable passenger cabin and showed signs of being a commercial success at its 1952 debut.

Supporting paragraphs: Alan Samuel Butler De Havilland Comet
Answer: Hertfordshire