Short answers, pulled from the story.
John of Lancaster and Humphrey, Duke of Gloucester jointly assumed the role of Lord Protector for Henry VI on the 5th of December 1422. Their joint tenure concluded on the 6th of November 1429.
Richard Plantagenet, Duke of York, served three separate terms as Lord Protector between April 1454 and December 1460. His first term began on the 3rd of April 1454 and ended in February 1455 before he returned to power on the 19th of November 1455.
Oliver Cromwell held the title Lord Protector from the 16th of December 1653 until his death on the 3rd of September 1658. The Instrument of Government established in 1653 defined him as head of state and government during the Interregnum.
The title fell out of favor after 1659 due to its strong association with republicanism following the Stuart Restoration. Political leaders found the term distasteful because it evoked memories of executed kings and revolutionary governments.
Lord Protector served as a translation of Latin Advocatus describing temporal lords protecting church interests while monarchs acted as protectors of secular aspects within ecclesiastical structures. These arrangements allowed secular rulers to manage church property while respecting spiritual autonomy rather than governing entire kingdoms.