How do you address letter to david Cameron?

Answer 1

You can be formal or less formal:

The formal approach is to address it to: The Prime Minister, 10 Downing Street, London SW1 and start the letter "Dear Prime Minister".

The less formal approach is to address it to: The Rt Hon David Cameron PM, 10 Downing Street London SW1 and start the letter "Dear Mr Cameron"

Addressing him as "Dear Dave" probably won't get a reply.