I was pretty lucky that I was in London during the right season, in the summertime you can barely move for outdoor screens. Theres a rooftop theater that they set up in Shoreditch, Stratford and Packham / Backyard Cinema in Camden and the Luna Cinema that projects classic movies wherever they can. Also, theres a little one under the Westway at Portobello road which is right next to Boom Burger! Make a night out of it, grab a burger from Boom Burger and then watch a movie. Thats exactly what I did.