If I might just interject this attempt at unifying our voices, please consider the following.

Entitlement, right, privilege, what you may call it, the notion that you can do something without fear of contradiction or sanction ( e.g., a size 12 in the aft area) really only matters when you are faced with someone who wishes to question your entitlement, or right, etc. That is, if you never marched a day in your life, you can still walk into one of a dozen stores and purchase an Argyle and Sutherland Highlanders regimental striped necktie. And you may wear it with pride, impunity, or even a striped shirt. The proof of this is the number of such ties sold and the number of places selling them- all in the United States, where an A&SH reunion could be held in a jury room. Nobody much questions it, even if they do know the name of the regiment.
But wander into just about any corner of the UK wearing a tie of blue and red stripes and you'd better be prepared to discuss when and where you served in The Guards ( Sorry, Jock, please supply the correct formal name) or why you are wearing their tie otherwise.

Similarly, I can wear my Buchanan kilt which I bought on spec on eBay and haven't yet sold again, or my Ancient Campbell one, or my Dress Gordon one all day long here on the wrong side of the tracks. Should I encounter a Buchanan or an ancient Campbell, or even a Weathered Stewart, I am prepared to talk with him about the relative beauty of the tartan and maybe even about my claims, real or fancied, to a shared heritage, respect, or affiliation.

BUT MARK THE RUSTIC, oops, I mean, but should I wander into the drawing room of the Duke of Argyll, or the chief of just about any clan, I would count on either wearing something other than his clan tartan, or having a pretty strong case for the actual unquestioned affiliation between myself and my host. ( I would point out that I don't think many of those Ladies and Gentlemen would be so rude as to upbraid me or criticize my dress, but I would imagine there would be some headshaking on the chiefly pillow later that night.)

I believe the expression is "not on", which is one of those ideas that are both stronger than law and absolutely unenforceable. Going to a man's party wearing his monogram, or standing in front of him and calling yourself by his name would hardly be illegal, or punishable by the law, but it would not be very polite, now, would it?