Addressing a letter to an MP throws up a lot of doubt and confusion about the proper etiquette. However, addressing a letter to an MP is straight-forward as long as you know whether the MP in question is a member of the Privy Council or not.
Write the MP's full name on the envelope. You can follow this with MP but it is not essential. For instance, write John Howard MP or Mr Howard MP. However, some letter experts advise just writing the MP's name such as Mr Howard or John Howard on the envelope.
Address a present or former government minister or Prime Minister with the term Right Honourable. Shorten this to The Rt.Hon followed by the person's full name (such as David Cameron).
Write "Dear Mr; Mrs; Ms or Miss" followed by the MP's surname at the head of the letter. For instance, you would write Dear Mr Benn to the MP Tony Benn, even though he is a member of the Privy Council. This would be the same for all MPs and ministers.
Send the letter to either the MP's constituency or the main London Parliament address: House of Commons, London, SW1A OAA.
If you email the MP treat it as a written letter in terms of style and format. Include your address at the top right of the page. Write the MP's address underneath yours, on the left. Start with the format "Dear Mr/Mrs".
You might have to wait a long time for a response to your letter if you send it to the House of Commons address, particularly during the summer recess. It is better to find out the MP's constituency address and send it there for a faster reply.