The site liveinternet.ru/users/4211284/post323988759/ has this about her (very approximate reconstruction of Google translation of the original Russian) "...In February 1819, a 37-year-old general went to his father to London to ask permission to marry. His bride, Countess Elizabeth Ksaverevne Branitskaya, was already 27 years old. During her travels abroad, she met Mikhail Vorontsov, who immediately made her an offer. Eliza, as he called her, had a Polish father and a Russian mother, relatives of Potemkin. She was beautiful and had incredibly fascinating charm that made everyone see her beauty..."
George Dawe and Sir Thomas Lawrence portrayed her, presumably while she was in the UK.