its not too late to start of your 20. infact its the best time to start as you have lived a bit in university and have a back up plan if it all goes down the drain a few years from now.
to be a police chopper pilot you will need a commercial pilots licence anyway and probably an IR i would think. i believe the the RAF deal with the rescue service but could be wrong.
to fly a chopper you will need a different licence to flying a jet aircraft across the world (hele or fixed wing) so you will need to do some reasurch into what each requires and think of what sort of flying you would like to do.