Brass 1-piece buttons with their backmark written in raised lettering date from approximately 1790 into 1830s, when they rapidly fell out of public favor due to the advent of less-inexpensive (but equally decorative) hollow 2-piece brass buttons.
On such buttons, the term "gilt" meant gold-plated, and "plated" meant silver-plated. Your button was produced by the famous Sheffield Plate process. Your button was almost certainly made in Britain. https://en.wikipedia.org/wiki/Sheffield_plate