Crispin, it looks like we are talking about 2 different scenarios.
Scenario A. Customer asks an insurance company for a quote. Insurance company assesses the risk and then makes a decision whether or not to offer insurance to the customer. I agree with you Crispin that the insurance company has the right to not offer you insurance.
Scenario B. Customer asks an insurance company for a quote. Insurance company assesses the risk and then makes a decision whether or not to offer insurance to the customer. Insurance company offers insurance but with some T's&C's like limited to 5000 miles a year or no cover for greenlaning. Customer agrees to these conditions and goes ahead and purchases insurance. Now I know for a fact that if the customer exceeds the mileage allowance the insurance company still needs to provide at least 3rd party insurance by law. This means that the policy is downgraded from fully comp, TPFT, legal cover etc. to just the bare legal minimum which is 3rd party only if the mileage clause is broken. Vehicle use eg. business or motorsprot etc. may invalidate your all your insurance if you are not covered by the your insurance company and the authorities check this as I have been asked to provide proof of insurance additional insurance for business use when I was stopped while delivering pizzas as a part time job. They did a check and discovered that I only had SP&C. AFAIK greenlaning falls under the same category as limited mileage, you will still have 3rd party cover if you break this clause as its a public road, nod different from any other public road as far the law is concerned.
So my question is this, in scenario B would the customer still have have 3rd party insurance if they choose to drive on a greenlane? If not then how would the authorities check this? Where do modifications fit into all of this?