Time Out says
Whether you're a film student, a movie buff or simply nostalgic for the telly of your childhood, BFI Mediatheque is a great resource. More than 50,000 film and TV titles from the BFI National Archive are available to watch for free at special viewing stations. They're curated in themed collections with new titles frequently added. Anyone can use these ‘digital jukeboxes’, although only age-appropriate material is made available to younger users. Just turn up at the BFI on the South Bank on spec and enjoy.
Open Tuesday to Sunday 10.30am-9pm (closed Mondays except bank hols).