Below find information coming directly from a representative of the Charity Commission in answer to the question about mandatory reporting in the UK and should for all practical purposes be recognized as an expert's reply.
BTW, I noted that the information in the post below is similar to what has already been posted on this subject already. This means we have experts among us who knew what they were talking about in their posts regarding this subject. So I'm giving them a :-) !
There is no mandatory reporting in the UK although there is in Ireland. It's therefore open to individual groups and charities to decide whether or not they want to have voluntary mandatory reporting. A good example in the UK is the Catholic Church in England and Wales, which does and, by way of contrast, the Church of England does not. There are however requirements under charity law here for the trustees of a charity to take reasonable steps to ensure that the people they work with aren't exposed to undue risk - this is where we site safeguarding. The question is: if you are not reporting to the police, how are you identifying and managing risk?