Oh yes, it is fine.
They don't have to make a report if nobody else wants to. There is no obligation upon anybody to make a report in the UK save for limited exceptions.
The other problem is that it depends on the nature of the 'assault'. Quite a lot of sexual assaults really amount to unsuccessful passes and they are more appropriate for a letter of apology if any action really needs to be taken at all. If it is more serious than that then it calls for some form of action but there is no need to bring it to the attention of the police.
Can I clarify anything for you?
So because the accused has simply just had to apologies, and the nature of his work could mean dealing with vulnerable okay to just let him get back to his position of a ward Councillor. It seems typical but not entirely fair. Apart from lots of sexual comments he started smacking and grabbing her backside.
Yes, he can return to office.
There is no duty to report it.