To address a member of parliament in a letter, simply refer to them as Mr. or Mrs. followed by their name and "MP." The MP stands for "member of parliament" and is the honorific used for standard members.
If the member of parliament being addressed is a minister or member of a cabinet or a shadow cabinet, they are addressed as "Rt. Hon" before their name, which stands for "right honorable" and is a way to show respect to their high standing within the government. From an etiquette perspective, it is always good to refer to a member of parliament by their full title in formal letters or introductions.