Heathrow isn't the only airport near London so redundancy is already built
https://www.thelocal.dk/20250321/sas-cancels-flights-from-no...
For national security London and UK can scramble several other airports for important flights.
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".
Always has been, always will be.