If someone has shared a private sexual photograph or video of you without your consent you can report this to the police. In an emergency you can contact the police for assistance by dialling 999.
The police may be able to attend the scene of the incident to protect you from further abuse and arrest your abuser.
In non-emergencies you can contact the police by dialling 101.
See the legal guide Reporting an offence to the police: a guide to criminal investigations which provides more information on reporting an offence to the police, providing a statement, and the police investigation process.
Remember to take screen shots or print hard copies of the abusive posts or messages to show to the police.
Your abuser may later delete their messages or posts so your screen shots may be very important evidence.
Great source of information on the topic: https://rightsofwomen.org.uk/