Crimestoppers is an independent charity helping law enforcement to locate criminals and help solve crimes. They have an anonymous 24/7 phone number, 0800 555 111, that people can call to pass on information about crime; alternatively people can send them information anonymously via their Giving Information Form. You don't have to give your name or any of your personal details. They do not trace calls or track IP addresses.
Crimestoppers pay cash rewards of up to £1,000 if the information you give to Crimestoppers leads to one or more people being arrested and charged. The information you give them is passed onto the police in such a way that they do not know who you are; they have never given the police information about their callers. It's a simple and secure way of bringing criminals to justice, without anybody knowing where the information came from.