London currently has 7 international airports and operates as the world's busiest hub nearing 180 million passengers p/a. It's often the world's most visited city and a handy stopover for changing planes.
However it has only 8 runways in total, only allowed to run in daytime, 3 of which are the world's busiest single strips despite. This is due to huge problems building any new ones, despite available land a small village blocks it, and hundreds of thousands who live in the vicinities complain of increasing any noise. It's at over-capacity already, despite the demand booming.
It's taken over a decade to even bitterly argue which airport should get the new expansion (Heathrow, currently with 81 million passengers p/a and long the world's busiest international airport, now overtaken by Dubai), and now there's the appeals process to go. Meanwhile local airhubs in Paris, Amsterdam, Madrid, Frankfurt and even as far as Istanbul are poised to take the slack, particularly from Asia.
Note how in the last few years Heathrow, stymied by its lack of expansion, has fallen from second busiest to 7th.
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Paris and Istanbul currently see in 110 million to their airports, whilst Madrid Barajas has capacity for over 100 million (and Istanbul 200 million when fully completed)