Does anyone know the legal basis that HAL is telling the airlines to stop selling tickets? Could this be considered interference in contracts?

Presumably HAL has advice it can do this, and no doubt ultimately decides what capacity the airport can operate at however previously they have told airlines to remove % of flights, now they are saying dont sell tickets/don't offer your services to customers.

I wonder if HAL is in the unique position it can dictate to its customers what they must do, but there is little or no regulation to force HAL to operate the airport in the national interest.

(An ill-defined term but you get what I mean - given the strategic importance of Heathrow what obligations does it have to operate in a competent manner to not do harm to the UK economy?)