As formal as I can get with what I own is a black Braemar (Barathea wool and silver buttons) w/ matching waistcoat. Add either a silver or black cravat and that will suit for about anything I attend. If I were going to a truly formal affair here I would rent a PC.

I like the look of the doublets but since I am one of those folks who generates enough heat to power a small country I'd roast in one. Unless I were invited to a formal event at Holyrood palace I'd probably wear one but not likely for anything else.