I didn't bother to send them a letter. As soon as I was declared BR I started answering my phone to them (something I had avoided for months), I then verbally told them my BR number and the Court and they were very polite and just thanked me and I have not had any calls since. I can't tell you how wonderful it felt to be able to tell them, I let them go through all their spiel and how much they wanted me to pay them immediately and then I dropped my bombshell.
After you've gone BR the OR will also contact all your creditors so don't think you need to bother with a letter to them.