Her article in thepeerage is here. The artist could be Allan Ramsay.
From bbc.co; enlarged by half.
Lady Charlotte's hands cover her waistline formed by her robes that function as an over-dress with over-bodice and over-skirt.
Keywords: 1785, Charlotte Jane Windsor, Baroness Cardiff, Countess of Bute, Marchioness of Bute, Windsor family, Stuart family, Marquise, British, bouffant coiffure, feathered headdress, bodice, square neckline, modesty piece, elbow length under-sleeves, engageantes, lace, full under-skirt, robes, over-bodice, elbow length flared over-sleeves, mantua, train, Sévigné bodice ornament, fan, court dress