If I was in search for a Royal Yacht substitute I'd choose University Flake. RY & UF aren't the same by any mean but they share some important common features, like strength level and plum flavouring.
GH&Co Kendal No7 shag may be even closer substitute because of its cut, but it lacks any sweet flavouring which puts it lower in the substitutions list.
I'm a bit surprised to see Royal Yacht being deemed by many here as a very strong blend, on par with Irish Flake or 1792. Never noticed anything special about its strength. A healthy dose of Nicotine, yes, but no more than that. I put RY into the Medium-High, not Strong category.