An insurance policy is always (other than when specific 'short term' insurance cover is taken out) an annual one. That applies even when credit is provided, enabling monthly payments to be made.
For example, a driver might be quoted £1200 a year for his insurance but be invited to pay it a rate of £100 per month. [I'm omitting any supplements added for monthly payments here in order to simplify my example]. So the deal is "You pay us £1200 and we'll provide you with insurance cover". If the car is then subsequently written off after only a few monthly payments have been made, the original deal still stands. i.e. the insurer person is obliged to pay the full £1200. (So if the accident occurs after two monthly payments have been made, the insurer will say "You still owe us £1000. Carry on making the monthly payments and then, when we come to pay out, we'll deduct the remainder of what's owing from what we pay you"). It's a perfectly normal insurance contract.