The compelling reason for publicity would be to cause the attack teams to abort their missions. That is what usually happens when pre-attack indicators get spotted. It is easier than finding safe houses and much easier than fending off an attack-in-progress.
If they actually found a safe house or an attack team heading for assembly, security forces would prefer to roll them up quietly without any publicity at all. Things might sometimes work out that way.
They needed an explanation for why they closed the road. Canal Road in that stretch has no sidewalks or shoulders.
I am wondering where this woman who supposedly saw the violin case was walking as the primary walking trail there is the towpath which is across the canal from the road and several feet lower.