For the sake of completeness, and because it calls itself a London airport, we include Southend. It’s about 50 minutes from London
It is a very small airport, with a few flights to Amsterdam and Alicante on easyJet.
There is a rail station just nearby.
Southend is on the coast, well, the Thames estuary. It’s a city most famous for its extremely long pier. There are many different methods to get to central London.
If you want to see a typical UK seaside city, Southend may be the place for a day trip.