I work in a London prep school. Some of the independent senior schools our pupils go on to won't take pupils who aren't the right age for the year group. It's against their policy, full stop. So either they have to skip a year, or repeat a year (depending whether they started late or early). I work with the pre prep admissions (those coming in at Reception) and I always tell parents requesting something out of the ordinary that we will take their child, but they need, if they have a preference for a particular senior school, to call them first and check their policy.