Help - I want out!

The recent furore about Brexit has led to the resignation of an MP.  Except that an MP cannot resign. Rules going back to the early 17th century dictate that the only way a sitting MP can lose his or her seat is by death, disqualification or expulsion. Another long-standing rule is that you cannot be an MP if you hold an office of the Crown, so various purely theoretical Crown offices have come into being to enable MPs to apply for them and thus disqualify themselves.  The most famous of these is the Crown Steward and Bailiff of the Three Hundreds of Chiltern - more commonly referred to simply as "the Chiltern Hundreds".