Always thought of slacks (yes, fairly dressy) as pants which could be worn with a sports coat or sometimes a blazer- which was a fairly dressy jacket like a suit coat. The blazer usually had brass buttons. Sorry for the definition of a definition kind of a sentence, but I have no idea what they are called in Britain.

I also remember that grown men who wore short pants in rural or small town South were whistled at by male locals and perhaps attacked if they went into a bar. Only a Yankee or foreigner would expose their legs in a public place.