As answered elsewhere - CPS do not need the victim's copnsent to proceed with a prosecution. You have been assaulted and a crime has been committed.
However, if you will not voluntarily give evidence all that can happen is that you are forced to court to give evidence and, if you still refuse, you can be treated as hostile and questioned as to your original statement.
Why do you want to withdraw? - if you have been intimidated let the police/CPS know before you finally decide to withdraw - they may be able to help.
If there is no real reason to back out then reconsider - those responsible will do it again and someone could die!!