The format of the playoffs during the weekend is two singles on Saturday. Then on Sunday, the two reverse singles though if one country wins both Saturday's singles and the first reverse singles on Sunday the second reverse singles isn't played. The doubles match is after these and gets played whether ot not it's a deciding match.
GB's singles players are likely to be Johanna Konta and Katie Boulter, Kazakshtan's Yulia Putintseva and Zarina Diyas. The doubles players could depend as to whether it's a deciding or dead match. Heather Watson is GB's strongest doubles player, Harriet Dart and Johanna Konta fairly close to each other in doubles rankings. Kazakhstan's doubles players probably Anna Danilina and Galina Voskoboeva (who is a doubles specialist and on a par with Heather Watson).