On whose authority?

BAA is the private (foreign owned) monopoly owner of most of the country's major airports. It may be colluding with the government to secure a third runway at Heathrow but it is very definitely not a public authority. It was privatised in 1986.

But according to the BBC:

The British Airports Authority (BAA) is calling for a high-speed rail link to Heathrow to complement a third runway at the west London airport.