I recently saw the Paddington Bear film for free at the Scoop. It's an excellent set up and you can't beat the location! Throughout the summer there are lots of free events happening - a big screen set up to watch Wimbledon, theatre and films.
I would definitely recommend either bringing something to sit on or renting one of the cushions available for the and bringing a blanket as it can get very chilly sitting by the river for a long period of time. The Scoop has a1000 person capacity but it's definitely best to arrive in plenty of time as the events aren't ticketed.