London like most major cities already has an airport. However, expanding airports means addicting a runway ~1.2km from existing runways which causes more noise issues as people are now effectively 1.2 km closer to the airport.

So, sure expanding existing airports is an option to consider, but doing so has a real and measurable negative impact on property values.

PS: I am not trying to promote a new airport further from London just saying such a thing is an option for London and most other major cities.