Possible Clues:

• U.K. carrier, once
• The first airline to use “Speedbird” as a call sign
• Former Heathrow-based flier
• UK airline
• Former Heathrow lander
• Airline created by an Act of Parliament
• Airline mentioned in the first line of the Beatles' "Back in the U.S.S.R."
• U.K. carrier of old