CPL or rotary ATPL theory exams passed. The CPL endorsement on your licence. Not sure if you'll have to do a full course or just set amount of training and type rated on say an R22 to be able do the CPL flight test if there isn't an option of an easy conversion for you.
Best advice is send an email to SRG at the CAA and give them all pertinent info and ask them exactly what you'll require. LASORS 08 should guide you.
Police and air ambulance not likely to employ you with less than 2000hrs, and multi experience, and not many jobs about.
Offshore, you'll need the JAR CPL/IR as minimum.
Lots of hoops to jump through i'm afraid.