I think the material I've seen in a tartan sample book, from one of the leading mills (I can't remember which), is the actual tartan used for MOD kilts. I believe it was listed at 18oz but I could be wrong.

It looks very different from civilian tartan, having a hazy indistinct look more like a wool blanket. I believe that the blanket-looking very heavy stuff is only used for Other Ranks' kilts, and that Officers etc use a tartan that looks like the normal 16oz civilian stuff.

I'm sure it wouldn't be difficult to find the mill that makes that heavy OR's tartan for the MOD.