Based on the dress, I believe this could be Charlotte Percy Ashburnham's daughter Charlotte or another of her seven daughters.
This dress does not have the loose folded appearance of a 1794 dress, but it does have the neckline and skirt of a dress from the late 1810s.
Keywords: Hoppner, British, straight coiffure, over-dress, square neckline, quarter length tight notched sleeves, Empire low waistline, belt, A-line skirt, wrap