I think the Mess Kit is "uniform/highly regulated civvies" debate is a grey area concept. Mess Kit is kind of both all at the same time.

Mess Kit is not provided by the military. One has to purchase one's own. Therefore, it's civvies. It is, however, an order of dress in the dress regulations with specifics as to how it's worn, therefore it's a uniform. Confused? Well, you should be. As I said, it's kind of both.

It's in the military dress regulations as a uniform, but they are purchased from various private tailors or uniform suppliers, so there can be a bit of surprising amount of acceptable variation in material, cut, workmanship, colour, etc. This can be seen sometimes when people who are theoretically wearing the same mess kit are seen side by side. Some red serge are more red than others. Some "old gold" embroidery will be very different from someone else's. This can be sometime's very readily apparent if you catch a young Second Lieutenant beside a General. If the 2Lt tried to save a few quid on his mandatory purchase of Mess Kit, he could potentially look quite poor compared to the General who, by this point in his life, has no financial worries. But, as long as he's in the required "pattern", he's okay and it's doubtful the Adjutant will have any issues with him - at least with regards to dress. What exactly a subaltern is doing hovering around a General Officer for in the first place is another issue!

Does that help?