London is by far my favorite city, and many people in Honolulu apparently feel the same. An old poll (5-6 years ago) said that 50% of polled people would go to London on that flight if it existed. The airlines are quite definitely aware of this, as Hawaiian Airlines responded a year later by saying they would make a direct flight. But it never happened, and no one has heard anything from them on the issue since. (This was more than four years ago.) I have considered the following:

 - Capabilities of planes (More than capable of making the flight)
 - Popularity of flight (The poll)
 - Flight restrictions (None)

After my pondering, I have concluded that there should be no problems connecting the two popular destinations. So why are there no flights? They would mean certain profit for the first airline to offer the journey. (They could charge whatever they wanted to.)