The pop star brought out the British singer in London Thursday evening for the popular acoustic set she's been doing during the "Eras" tour. It frequently includes songs she rarely performs and fans got an extra special treat this time.

Taylor and Ed proved to be quite "The A Team," as the BFFs sang a mash-up of "Everything Has Changed," "End Game" and "Thinking Out Loud" for surprised Swifties.

TS has performed more than 100 "Eras" tour shows, and sometimes special guests join her onstage ... including Hayley Williams, Ice Spice, Phoebe Bridgers, Sabrina Carpenter, Gracie Abrams, Jack Antonoff, Aaron Dessner and HAIM.

Taylor referred to Ed as her "second brother" while introducing him to the crowd ... putting an emphasis on their special bond.

She added ... "He works so hard and he's on tour right now and he's probably so tired, but he wanted to come and play for you and do this for all of us. So, give it up again for Ed Sheeran."

Taylor and Ed have collaborated a number of times over the years. Not only did Ed open for Taylor on her "Red" world tour, but they've recorded several duets ... including "Everything Has Changed," "End Game," "Run" and "The Joker and the Queen."

Ed wasn't the only notable name in attendance at Wembley Thursday evening ... as Idris Elba, and a couple of UK comedic celebs Ambika Mod and Amelia Dimoldenberg were also spotted in the crowd.