The Portcullis with a crown on top has come to be accepted as the emblem of both Houses of Parliament. As with many aspects of parliamentary life, this has developed through custom and usage. Henry VII and VIII used the Portcullis as a badge and probably used it throughout the new Palace of Westminster. In 1996 the use of the crowned Portcullis was formally authorised by license granted by the Queen. This allows both Houses to use it and to regulate its use by others.
Further information on this subject can be found from the following link.
Parliamentary Copyright - The Crowned Portcullis