The decision may not be fair as only relying upon what an MRI states does not provide the full picture. It is not illegal for an insurance company, such as Standard Life to deny or terminate your claim based on their interpretation of the medical evidence that has been provided, but you can challenge this decision by having your case reviewed by a disability lawyer who can assess your chances of success in not accepting this decision.
One other point is that if the Car Accident case is still open, you should discuss whether you may be able to claim for loss of income as a result of this termination of employment, even though it occurred many years after the accident in question. If your car accident case has already been settled, it would be too late to raise the issue. Consult with a lawyer experienced in motor vehicle accidents to discuss this specific point.