Assisting the Police in thier enquiries is always going to help, saying that, it won't be easy.
Depending what happens in terms of charges brought against him, and whether he pleas guilty or not guilty, will dictate if you have to appear in court to give evidence.
It'll help if I explain the process (sorry if you already know)
1) The Police will build a case file, which contains evidence. This involves statements, officers notes, intelligence, past convictions, etc.
2) Your dad may or may not be released on bail, or conditional bail.
3) This "case file" is then passed the to Crown Prosecution Service who will review the case and decide if it is in the public interest to continue (More than likely yes in this case)
4) Your dad will be given a date to appear in court to answer the charges made against him, he can plea guilty or not guilty.
5) If he please guilty then no further parties are likely to be questioned, and he will subsequently be charged.
5 pt 2) If he please guilty then all parties giving evidence (victims and witnesses) will go and provide evidence in the court.

Long winded process I know..

The Police may or may not ask for a statement, i don't know as I'm not the Officer In Charge. They will make that decision. However you can always supply a statement which will then be taken into consideration.

Important thing to remember, you are NOT obliged in any way shape or form to provide any evidence if you do not want to, regardless of how much the Police try and persuade you.

Get in touch with the Victim support unit, the police can provide you with their number. ( I found them very helpful)

Good luck with whatever path you choose and I hope all is well.

Alex