This has been debated over and over. Go see NPAS forums etc.
On first glance, it might look tremendously cheaper than "real" chopper. But, please, has someone made a realistic calculation of the cost and compare it with the capability of such UAS system? There still needs to be quite a number of people on the ground, to operate and maintain it, there are operational limitations (can someone explain further how to operate in controlled airspace which is plenty in UK?) I seriously doubt this (or any comparable) UAS will last thousands and thousands of hours, so please include realistic lifespan in calculation too.
Am sure UAS will come in bigger numbers (Canada RCMP "mounties" operate by far the largest police UAS fleet, not to mention Turkish police, that runs serious UAS outfits, albeit in a way that might not work in EU airspace/legislative standards...) but are not (yet) a direct replacement for manned police aircraft.