Taylor Swift stunned an audience in London on Thursday when she made a surprise appearance during HAIM's concert at the O2 Arena.The 32-year-old pop icon took the stage with a mic in hand as the audience went wild, and Swift introduced a «mash-up» performance that she sang with the trio of singing sisters.“I haven’t been on stage in a very long time. It’s nice [to be back]," Swift declared.«When I heard my girls were playing in London at the O2, I thought, 'I'm gonna have to see that,'» she continued.