BARCELONA — Antonio Banderas has been added to the lengthening cast of Woody Allen’s next pic, a still-untitled project set to roll this summer in London, according to producer Mediapro.The Spanish actor joins the already-announced Naomi Watts, who will co-star with “Slumdog Millionaire” discovery Freida Pinto, Josh Brolin and Anthony Hopkins. This will be Banderas’ first gig with Allen. Film will be produced by Letty Aronson, Steve Tenenbaum and Jaume Roures, a principal at Barcelona-based Mediapro, which will also finance the movie. Aronson’s Gravier Prods. co-produced “Vicky Cristina Barcelona” with Mediapro. The London project kicks off a three-pic co-production deal between Allen and Mediapro.