Conversation so far:
how many international airports are in the uk
There are 22 international airports in the UK.
how many international airports are in london
London serves as the largest aviation hub in the world by passenger traffic, with six international airports.
what is the biggest airport in london
London-Heathrow, which handles over 80 million international passengers annually, is the largest airport in the UK.

Question: when was the airport opened
Self-contained rewrite: when was the london-heathrow airport opened