London has 5 airports, Heathrow, Gatwick, Stansted, Luton and London City. The biggest airport is of course heathrow. Over 65 million passengers travel to or trough London a year. Stansted is Ryanair's home airport and it fliesfrom many locations in Europe.
When flying to London, remember that England has very strict airline policies, so you might have to wait in lines to get trough security. That is especially important if you have connecting flights to get to.