By law you must tell the police in writing six days before a public march if you are the organiser.
Tell the police the:
date and time of the march
the names and addresses of the organisers
The police have the power to:
limit or change the route of your march
set any other condition of your march
If you arrange a march at short notice, you must still tell the police as soon as you can.
The police can also:
change the location
limit how long a rally lasts
limit the amount of people who attend
stop a sit-down protest if it blocks road traffic or public walkways
If there is no march involved
If there is no march organised as part of your protest, you do not have to tell the police.
For further information on protests and marches - letting the police know can be found on GOV.UK's website.