The offences in the Act were originally high treason under the Treason Act 1661 (later the Treason Act 1795), and consequently the penalty was death. However it was found that juries were often reluctant to convict people of capital crimes, and it was thought that the conviction rate might increase if the sentence was reduced to exile to the penal colonies in Australia (the penalty today is life imprisonment). Consequently in 1848 three categories of treason (all derived from the 1795 Act) were reduced to felonies. (This occurred during a period when the death penalty in the United Kingdom was being abolished for a great many offences.)
It is treason felony:
In 2001, The Guardian newspaper mounted an unsuccessful legal challenge to the Act in the High Court, alleging that the act "...makes it a criminal offence, punishable by life imprisonment, to advocate abolition of the monarchy in print, even by peaceful means..." They sought a declaration that the Human Rights Act 1998 had altered its meaning so that only violent conduct was criminal. The court held that this was a hypothetical question that did not deserve an answer, since they were not being prosecuted. The case eventually went to the House of Lords on appeal in 2003. In a unanimous judgement the House agreed that the litigation was unnecessary, but most of the judges nevertheless agreed with (and none disagreed with) Lord Steyn's view that "the part of section 3 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 section 3 could survive scrutiny under the Human Rights Act is unreal."
The wording of the relevant section of the Act is:
3. Offences herein mentioned declared to be felonies
If any person whatsoever shall, within the United Kingdom or without, compass, 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, or of any other of her Majesty’s dominions and countries, or to levy war against her Majesty, within any part of the United Kingdom, in order by force or constraint to compel her to change her measures or counsels, or in order to put any force or constraint upon or in order to intimidate or overawe both Houses or either House of Parliament, or to move or stir any foreigner or stranger with force to invade the United Kingdom or any other of her Majesty’s dominions or countries under the obeisance of her Majesty, and such compassings, imaginations, inventions, devices, or intentions, or any of them, shall express, utter, or declare, by publishing any printing or writing ... or by any overt act or deed, every person so offending shall be guilty of felony, and being convicted thereof shall be liable ... to be transported beyond the seas for the term or his or her natural life.