I don't believe that temporary signage, such as the road closed and no entry signs in Perry Vale have any legal significance. They do not comply with the Traffic Sign Regulations and, I suspect have generally not been put in place under a traffic order. Consequently, the Police car wouldn't have been able to do much about the drivers involved. Are you sure the car wasn't also taking advantage of the fact that the road wasn't actually closed despite the signs?

Throughout the South Circular closure, there was a 'No left turn' sign at the lights at the end of Honor Oak Road. The only way to access Waldenshaw Road and the Pearcefield Road car park from Honor Oak Road was by ignoring the sign, which those in the know did. I suspect it was merely put there to discourage the use of Waldenshaw Road / Manor Mount / Davids Road as a blockage bypass.