Her article in thepeerage is here. She was also known as Countess Ashburnham. She was Viscount George Ashburnham’s second wife and bore 12 surviving children.
Upgrade image posted 1 August 2018 from the British Museum Web site; removed monocolor tint.
Unlike this image, that is most likely mis-dated and mis-identified, this portrait shows Lady Charlotte dressed for the era of fashion chaos of the early 1790s. I don't know what to call her head cover - a jumbo turban? Big coiffures demand big enclosures.
Keywords: 1797, Wilkin, Hopner, Charlotte Percy (1776-1862), Charlotte Ashburnham, Viscountess St Asaph, Countess Ashburnham, Percy family, Ashburnham family, Viscountess, Countess, British, curly coiffure, high turban, bodice, vee décolletage, long tight upper puffed sleeves, chemise, trapezoidal neckline, waist sash, bows, close skirt