The title Supreme Governor of the Church of England was settled in the Elizabethan legislation of 1559 and every subsequent monarch has held it. In practice it is an office of constitutional form rather than of doctrinal authority. The monarch formally appoints archbishops, bishops and deans, acting on advice; the Church of England makes its own decisions through its General Synod, whose measures then require parliamentary approval; and no part of the arrangement gives the holder a say in what is taught.
The relationship with Scotland is different in kind and is often assumed to be the same. There the monarch is not governor of anything but is a member of the Church of Scotland, which is presbyterian and holds that Christ is its only head, and the accession declaration includes a separate undertaking to preserve its settlement. Charles III made the accession declaration in September 2022 at the Accession Council, in the form the statutes require, including the promise to maintain the Protestant succession and the security of the Church of Scotland. The coronation oath, taken at the service the following year, is a further and separate set of promises, and the two are frequently confused in reporting.