Yes and no. The vast majority of commercial flights what would have landed at Heathrow today won't be landing at a different London airport instead. There isn't spare, redundant capacity for that. Instead the flights will be cancelled.
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".
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.
https://www.thelocal.dk/20250321/sas-cancels-flights-from-no...
https://www.bbc.co.uk/news/articles/cly6e24q8glo