Gregor, I'm no authority on the issue, but I don't think an LP is necessary to bestow the title POW on the Heir Apparent (male) to the Throne. It's a title that has been given to the Heir for hundreds of years. It's not automatic, of course, but I would think the word of the monarch is sufficient. But, on the other hand, I could be totally wrong here.
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I am sorry if I missed this, but have we seen an official document (Letters Patent) creating the Duke of Cambridge as Prince of Wales, or has there just been the announcement in the televised speech?