Lady Arden was sworn in as a Justice of the Supreme Court on Monday 1st October and the ceremony may be viewed here. Lord Kitchin was also sworn as a justice at the same ceremony. Biographies of the 12 justices - 9 male and 3 female - may be read here.
Lady Hale of Richmond - now President of the Supreme Court - was the only female to be appointed to the Supreme Court's predecessor, the Appellate Committee of the House of Lords.
Legal Cheek 17 September 2018
Interestingly, the Supreme Court of Ireland sat with its first female majority in 2000 - Dalton v Governor of the Training Unit  IESC 49 (29 February 2000) - Denham and McGuinness JJ sat with Hardiman J. [h/t - Irish Supreme Court Review -via Twitter].
The next planned change at the UK Supreme Court will follow the retirement of Lord Sumption in December. Lord Justice Sales will join the court in January 2019.