British Officers formal mess dress is uniform, regulated by the Mess Committee under the Commanding Officer of the establishment, who institutes rules set out under Kings regulations and varies dependant upon which mess the wearer belongs to; the Officers' Mess, Sergeants' Mess, Senior Non-Commissioned Officers, Warrant Officers and, the Junior Ranks' Mess. In some cases mufti may be all that is required, though usually only in the Junior NCO's mess.
I think the Navy and RAF may have different requirements.
Athol Highlanders, though a 'private army' would fall under King's Regulations at time of conflict, therefore are considered regulars and as such are governed by Kings' Regulations.
The belt over the waistcoat is in place of the stable belt worn by troops of the line, denoting the regimental 'attachment.'