Although King George III of Great Britain did respond to a Loyal Address using the personal pronoun I:
I thank you for this dutiful and affectionate Address. The satisfaction which you express, on the intended marriage of my sister ...
[A Parliamentary History, Hansard, 1765–6]
some thirty years later he had progressed to using the Royal We:
By the KING,
Whereas, on the Twenty-ninth Day of this Instant Month of October, divers Persons, riotously assembled in different Places in Our City of Westminster, proceeded to commit certain daring and highly Criminal Outrages, in gross Violation of the Public Peace, to the actual Danger of Our Royal Person, and to the interruption of Our Passage to and from Our Parliament: We therefore, with the Advice of Our Privy Council, in pursuance of an Address from Our Two Houses of Parliament, do hereby enjoin all Magistrates, and all other Our living Subjects, to use their utmost Endeavours to discover and cause to be apprehended, the Authors, Actors and Abettors, concerned in such Outrages, in order that they may be dealt with according to Law: And We do hereby promise, That any Person or Persons, other than those actually concerned in doing any Act by which Our Royal Person was immediately endangered ...
[Journals of the House of Commons, 36 Geo III (1796)]
Now, it's plain that a Proclamation is more official and weighty than a reply to a Loyal Address. But the King obviously knows he's only one person ("Our Royal Person") so apart from that one person, why does the King refer to himself in the plural? Who or what is the "other" who is included in Our and the Royal We? The Proclamation would have been just as effective if "My Royal Person" had appeared with capital letters.
As a side issue, it's interesting that proper nouns like George, October and Westminster get italicised, presumably to differentiate between them and common nouns which get capitalised (rather like modern German). But Our is always capitalised as well.