Addressing a letter to a member of parliament always follows the same general form, regardless of country: address envelopes with "Mr./Miss/Mrs./Miss [First name] [Last name], MP." In the letter itself, "Dear Sir," "Dear Madam" or "Dear Mr./Miss/Mrs./Miss [Last name]" is acceptable.
You may use the more formal prefix of "Honorable" as well; however, take care to include a full name, such as "The Honorable James Bentley, MP" or the standard prefix, "The Honorable Mr. Bentley, MP." In some parliamentary countries, the "Honorable" title is reserved to denote MPs who have been promoted to an executive council. If in doubt of its proper usage, it is best to avoid the "Honorable" title entirely. Conclude letters with "Yours Faithfully" (formal) or "Yours sincerely" (semi-formal) followed by your signature.