The admin fee is reasonable.
This is one of the few occasions where I think that talking to a supervisor/manager might help, the registration plate should not make any difference to the rating.
Agreed.
Incidentally, the only part of the car that you can't really steal is the registration mark... (as opposed to the physical license plate).
In general, there are three reasons why a mod will cause increase in premium:
1. Car is more powerful / can go faster / more dangerous etc - e.g. ECU remap, increased turbo boost
2. Car is more desirable to thieves - e.g. alloy wheels
3. There is a statistical connection between this specific mod and likelihood of a claim (i.e. suggesting a behavioural pattern, such as more aggressive driving style) - e.g. fitting a large wing-type boot spoiler
Apparently, in this case the RAC wrongly used clause 2. They should have used clause 3 (i.e. suggesting that owners who have private plates are less careful drivers). This would probably prove to be statistically incorrect, but this argument would make sense on a logical level if not the factual one, as opposed to clause 2 which - as already said by others - does not make any sense al all...
Incidentally, when I got my private plates, Aviva did not charge any admin fee when changing my policy, not did they increase it. They also haven't increased it when I installed COMAND (which - according to Aviva - did not require a change of policy or even notification, even though I did notify them anyway - and according to them my retrofitted COMAND will be fully covered!)