How does the police think it is a good idea to make the raid when no one is inside. The article said that a lot of people go in and out everyday and no one was arrested.
Having people there raises the probability of either injury or evidence destruction/contamination. They probably know who those people are anyway, so once they have the evidence they can go after the people one by one at leisure.
My thought as well I suspect they thought they would catch a bunch of trafficked people well at least they didn't Get Prti Patel down for the photo op.