If your child has a final EHCP that names a school, you do not need to apply through school admissions. The EHCP names the school your child attends.
If you have recently askes for an EHCP assessment, or the assessment has started but is not concluded, you should still apply through school admissions in case no final plan is granted.
If a final plan is made later on, the school named in the EHCP overrides any school admissions place offer made for your child.