Yes, you can report fly-tipping anonymously. Southend Council allows you to report incidents of fly-tipping by filling out an online form, and you do not need to provide personal information to make a report. However, keep in mind that if evidence is required for enforcement action, they may ask for a witness statement if you were present during the incident.
It's important to provide as much detail as possible about the incident to assist the council in their investigation. If you can safely take photographs or gather information about the offender, that can also be helpful in pursuing enforcement action.