This is how cop cars look where I live:
Ford Focus Mk2
Ford Focus Mk2 (Facelift)
Ford Focus Mk3
Fact is, police forces will have to replace their old cars with new so there will always be old cars mixed with new ones.
From where I live you can see all 3 models in use at the same time, I'm sure theres a few old Mk1's knocking about too.