No. No insurer offers a refund of your base allocation of miles if you don’t use them up. Nor do they roll over.
Miles accumulated in other ways may roll over into the next year, depending on your insurer. It’s always best to check your policy document first.The cost of these miles is based on your risk. So it’s calculated using factors similar to the ones that would normally be used for your premium.
Insurers tend not to offer money back if you go under your estimated mileage. If you drive less than the policy limit states, get in touch with your insurer to work out a more accurate estimate.