The Treason Felony Act 1848
section 3 makes it a crime to "imagine, invent, devise, or intend to deprive or depose our Most Gracious Lady the Queen, from the style, honour, or royal name of the imperial crown of the United Kingdom". One found guilty of such an imagination is "liable to be transported beyond the seas for the term or his or her natural life", though "Reference to transportation for life to be construed as reference to imprisonment for life or any shorter term".
It is worth noting that the act has not been deployed in a prosecution since 1879, and in a legal challenge to this law Lord Steyn said: "The part of s3 of the 1848 Act which appears to criminalise the advocacy of republicanism is a relic of a bygone age, and does not fit into the fabric of our modern legal system. The idea that s3 could survive scrutiny under the Human Rights Act is unreal." However that case failled to change the Treason act, and the there is an effort to repeal the Human Rights Act, though that is not going so well.