There should really be no difference to the premiums that you pay as a PDI compared to an ADI. The main difference between a PDI and an ADI is that the formet has still one more test to pass to get on the Official DSA register and this will show that the instructor has demonstrated competence in teaching students how to drive. Other than that,
PDI insurance premiums should really be more or less the same. Of course some people would prefer somebody who has got the licence to prove his competiveness and that could mean more business to the ADI as apposed to the PDI but insurance-wise, the risks are the same.
