Under the National Curriculum, Religious Education is a "statutory subject with a non-statutory programme of study". So all state schools are obliged to teach the subject but exactly what they teach, and the way that is taught, is up to the governors of each school to decide.
Faith schools, within the state sector, can be 'voluntary aided', 'voluntary controlled' or 'foundation'. In a voluntary controlled school the local education authority normally controls the admissions policy (but in a way which is sympathetic to the views of the governors). In a voluntary aided and foundation schools, the admissions policy is determined by the governors (in consultation with the local education authority). It is permissible for the admissions policies of all such schools to require that the child's family can demonstrate their commitment to the religious ethos of the school, e.g. by regular attendance at an appropriate church, synagogue or mosque.
Chris