Many Candidates who claimed they believed in the Sovereignty of the British people and the democratic powers of our Parliament before they were elected, betrayed those very rights when they got into Parliament. The British people therefore no longer trust any political party in Parliament, nor do they trust individual parliamentary candidates of those parties.
By committing the candidates of the Parliamentary parties to presenting and voting for the BDI Bill (when a majority of MPs is available to pass it into law) the BDI does not rely on trust. Those who sign the BDI commit to resigning if they are frustrated in their undertaking. By resigning on the issue of the Sovereignty of those whom they represent the BDI candidates will then seek a fresh mandate from the people (ie the Sovereigns!).
Many candidates of the Parliamentary parties who do not believe in the sovereignty of the British people have learned how to use the same words as those candidates who DO believe in (and will assert) that Sovereignty. The former will not sign the BDI, the latter will - so you can tell the difference.