The xx had been hosting their well-received “Day + Night” series at the O2 Brixton Academy and bringing out musical friends such as Cat Power, Sampha and Robyn to help them cover a few tunes. Last night, The xx brought out the potent Florence Welch to perform Jamie xx’s 2009 remix of Welch’s band’s cover of Candi Staton’s house tune You Got the Love, achieving pop equilibrium.