No BAA anymore its Heathrow Airport Holdings which have no shares so you could either buy the debt issued or invest in one of its owners but the share price will be made up of a lot more than Heathrow alone.
https://www.heathrow.com/company/company-news-and-information/company-information
"Our company, Heathrow Airport Holdings Limited (formerly BAA) owns and runs London
Heathrow Airport, Britain's aviation hub.
Heathrow Airport Holdings Limited is in turn owned by FGP Topco Limited, a consortium owned and led by the infrastructure specialist Ferrovial S.A. (25.00%), Qatar Investment Authority (20.00%), Caisse de dépôt et placement du Québec (CDPQ) (12.62%), GIC (11.20%), Alinda Capital Partners of the United States (11.18%), China Investment Corporation (10.00%) and Universities Superannuation Scheme (USS) (10.00%).
The company is subject to financial regulation by the
Civil Aviation Authority (CAA) and the
Competition and Markets Authority (CMA). In matters of safety and security we are regulated by the Government and CAA."