Is this a breach?
This sounds like normal campaigning
Campaigning itself is legal — posters with authorisation in the campaign period, permitted rallies, and ordinary canvassing are not breaches. If you are still unsure, send the report anyway and say exactly what you saw: the desk decides, and a report that turns out fine costs nothing.
Send a report anyway← Start again — every report stays alleged until a reviewer checks it. Checked reports go to the proper authority, such as the Electoral Commission or police. Nothing is published.