"Shut it down there and then" !! You could serve an art 31 notice but to try and false everyone to leave immediately is nigh on impossible, the RP cannot physically push them out of the doors, I have been in enough places to see how difficult it is sometimes for hefty great bouncers to get people to leave, there would be a riot. Even of you did manage to get everyone out at once (highly unlikely and again likely public order issues outside that the police would not thank you for), clickers recording numbers out (prosecution) against clickers recording numbers in (defence) is not likely to prove beyond reasonable doubt guilt. Count the legs and divide by two...really!!
I was involved in an exercise like this a few years ago, waste of time, as soon as we turned up with the police word got around and it was impossible to accurately carry out a click count, would be slaughtered in court by any half decent defence brief.
Finally with personnel levels these days, it's a struggle to get enough machines at a persons reported incident let alone personnel to carry out a head count. Don't wish to sound negative, just pragmatic.