You must send in a copy of your child's birth certificate when you accept, if you haven't already done so.
You can do this by scanning it in, or taking a photo of it on your smart phone, and emailing it to email@example.com.
Alternatively, you can post it to:
Bracknell Forest Council
If you've lost the birth certificate, you should accept the offer anyway. Order a new copy, and tell the Schools Admissions team what has happened. When the new birth certificate arrives, send a copy of it to the School Admissions Team.