In September it is the annual open House London event, where 600 buildings are opening their doors to everyone. I have no idea about most of it (I am sure it is easy to search) but in case anyone has a child as obsessed by theatres as mine currently is, just thought I'd let you know that the Theatre Royal in Stratford (E. London) is one of them. From 11am to 4pm you can go backstage on Sat and Sun 17th and 18th Sept. It is free.