[SideburnsOfDoom]

If by "national security issue" we mean "can the UK move military aircraft and important people" then no, Heathrow isn't key at all. There are other airbases and airports.

If we mean that a long outage would have economic impact and is hard to find the capacity elsewhere, then yes. As per grandparent post "take down one of Europe’s largest airports and global air travel".

[izacus]

Yeah well, not everything that annoys people and loses bussinesses money is a "national security issue", no matter how far the US overton window shifted on that matter.

[TheOtherHobbes]

A core goal of national security is protecting critical civilian infrastructure.

Always has been, always will be.