was that black tie/mess dress or no1 dress, that would make more sense.

Mess dress is different for each regiment but similar to posted photo, No1 dress is Blues which is what I got married in its a tunic with high colar worn with matching trousers, with red stripe, No2 dress is normall BEST uniform, and is a green tunic, worn with shirt and tie.
Blues is worn in the mess if you don have mess dress.
Kilt, Formal jacket (argyle/charlie) waistcoat, wing shirt and bow tie, is always acceptable in place of dinner suit