I completed my PPL, CPL and IR part-time, weekends only. Blackbushe Aviation were the only people able to accomodate my weekends only schedule. My own experience suggests that getting rusty after not flying for a few weeks is a bit of myth, but with such a constrained schedule as mine, a few weeks of no flying needed to be expected from time to time. For somebody in the Police, it will be heaps easier as you can more easily be accomodated with more weekday based schedule.