Air India is actually the LONGEST SERVING INCUMBENT carrier in the UK-USA market, having served LHR-JFK continuously for over 40 years now.

AI's authority to operate between the US and UK is obtained as part of US DOT order 69-6-155 which is extra-bilateral and predates the current Bermuda II agreement.

Remember, in the days of yore before long-distance flight became common, European airlines had to make fuel stops in India to get to the Far East. North American carriers still stop in Europe/Asia to get to India. At the time, India granted those fifth freedom rights to folks like BOAC and Pan Am in exchange for reciprocal rights from Heathrow (and most of Europe as well).