[TheOtherHobbes]

"National security site" is not a synonym for "military installation."

It means "critical infrastructure whose failure causes significant adverse effects."

The UK's main airport is absolutely that.

Your quote about 99% of air cargo not coming through Heathrow is made-up nonsense. The correct figure is closer to around 50% by volume and 70% by value.

https://www.heathrow.com/company/cargo

It's a major, major hub, not just for the UK but Europe, the US, and Rest of World.