Here's the question; Since they just arrived, have no clue what went on and didn't witness anything, what would the police do with them? Tell them to go home? Give them a number to call? Ask them to come down to the station politely? I googled the heck out of this, but didn't find anything.
Second question: Would they close the whole building, or just the area where the body was? It's an office block if that helps.
Googled: uk police procedures murder investigation, police procedures england, police procedures murder witnesses, various combinations of words.