SAMARITANS
The Samaritans have specific information about how to help and support someone online - http://www.samaritans.org/how-we-can-help-you/supporting-someone-online
You can call them at any time if you are worried for yourself or someone you know (It's a FREE UK number that will NOT show up on your telephone bill)
Call the Samaritans on 116 123
THE CYBERSMILE FOUNDATION
The Cybersmile Foundation is a multi award winning anti cyberbullying non-profit organization committed to tackling all forms of digital abuse and bullying online.
You can find information, help and support on their website at https://www.cybersmile.org/
Twitter provides several ways to report a cyber bully - please see their support website for more information.