Your local authority has a school admissions department and they are responsible for ensuring your child gets a school place. Since September 2013, Looked After Children (LAC) and children who left UK care on an Adoption Order, Special Guardianship Order or Child Arrangements Order (formerly Residence Order) have the highest priority. The authority which placed your child can provide a letter confirming your child's status.
The admissions code states:
1.7 All schools must have oversubscription criteria for each 'relevant age group' and the highest priority must be given, unless otherwise provided in this Code, to looked after children and all previously looked after children. Previously looked after children are children who were looked after, but ceased to be so because they were adopted (or became subject to a child arrangements order or special guardianship order) … immediately following having been looked after. Oversubscription criteria must then be applied to all other applicants in the order set out in the arrangements.
You can find the full admissions code on the DfE website.