You can call police on 101 (or 999 in an emergency) and there are also a variety of organisations who you can contact, both to make a report and also for help and support.
The Parents Protect website is part of the Stop it Now! campaign. It is an information and resources website which aims to raise awareness about child sexual abuse, answer questions and give adults the information, advice, support and facts, they need to help protect children.
This website is run by the Child Exploitation and Online Protection command and provides advice for parents on how to keep children of various ages safe online, as well as allowing you to report abuse online.
If you're concerned about a child, you can contact the NSPCC by calling 0808 800 5000. Their website also contains lots of useful information and advice, including how to spot the signs of abuse and what you can to help prevent abuse.