If you, (serving personnel), were to push this, possibly starting with the Insurance Ombudsman, and discrimination was proved I don't think it would help you much as the insurance companies would simply increase their rates, claiming actuarial experience required this. Insurance is an industry and not a profession. Just find an insurer that will cover you.
Regarding a car getting damaged on military property, the single biggest reason for dis-allowing a claim is non disclosure of essential information. In this case did you tell your insurer that you sometimes used your vehicle in conjunction with your work? That would normally be all that is required and may not always mean an increased premium, just depends on the amount of exposure. One reason some insurers don't want to cover you when your vehicle is on military property is because if you are hit by a military vehicle the insurer then has to deal with the MOD and not another insurance company/underwriter.