TV sitcoms rarely translate well to the screen (I can't actually think of any that do) and here's why. The world of the TV sitcom is a small world- usually studio bound- a step away from the proscenium stage, two steps away from the Punch and Judy booth. The real world barely impinges and even when the comedy is black the mood is cosy. The characters are Jonsonian humours- fixed in themselves and in relation to one another. They cannot change because to do so would break the format. Every episode must be like every other. The same motions must be gone through. We're longing to hear the catchphrase- and we greet it like an old friend when we do. Harold Steptoe can never leave his horrible father; if he did the show would be over. Alf Garnet can never learn from experience and modify his views; the show would be pointless if he did. Corporal Jones is absolutely not allowed to marry Mrs Fox- or at least not until the very final episode. The change in his status signals that the show has been cancelled. Something has finally happened in the lives of these people; finita la comedia.
Film on the other hand tells stories. It moves it's characters from A to B. Change is expected of them. They have an arc. At the very simplest level we expect the hero to defeat the bad guy and get the girl- something even comics with a fixed persona- like Buster Keaton- get to do. Sometimes the logic of the action demands they die. A sit-com character who cannot change makes a useless movie hero.
So in Porridge our hero Fletch is more acted upon than acting. He and the other regulars are funny men having funny things happen to them. They cannot move on, they cannot develop their relationships, they cannot have anything going on behind the eyes. There is a prison break. Fletch gets haplessly mixed up in it. The last quarter of the film has him and cell-mate Godber doing their best to break back into prison- not because the story demands this manifest implausibility but because if they got away- if they had to develop their relationship on the run- it would render the next season of the show impossible.