The market value derived from ISM-ABI will be recommended to you in the quotation page. You can adjust the amount you want your car to be insured for, which will be the the sum insured of your car, insured on an Agreed Value basis.
It is advisable to insure your car at current market value.
Insuring your car at on an Agreed Value basis means payment of claims in the event of (1) actual total loss, (2) the car being beyond economical repair or (3) theft will be based on the agreed value you chose and as stated in the policy.