Paying your car insurance off in one go is recommended as it will always work out out cheaper than paying in installments. The only potential downside of paying in one installment is that any alterations made to the policy will have to be paid in a lump sum as well.
Every insurance company will allow you to pay for your insurance policy in monthly installments. The catch is they often use a finance company (unless they are a big insurer who have their own finance people) so it ends up costing a lot more in the long run (normally 10% extra). The advantage of paying for insurance monthly is it makes it more affordable, especially for those who are on a tight budget and could not possibly save enough money to pay it in one go.
Another advantage is if you make a change to your policy such as declaring a modification, your monthly payment will just go up slightly instead of having to pay it in one go.