In printed British records, Welsh rarebit predates the macaroni cheese. The former first appears in 1725, whilst the pasta has to wait till 1769.
By the by, you’ll often come across a version of the Welsh rarebit in northern France, where it is known as “Le Welsh”. Of course, that part of France is only 22 miles away from England so the dish might have travelled at any time but my guess is that this will have Great War connections. I’ve not tried it so don’t know how it compares with the original.
