I had a strange situation a few years ago.

I was taking out Home Insurance with (IIRC) M&S. When they asked if I wanted to pay monthly, I enquired about their APR - this turned out to be in excess of 23%.

However, for some reason, the lady at M&S did then say that if I elected to pay monthly, then my premium would be reduced. In the event, the total of the (12 x monthly premium) was less than the annual premium, even with the 23%+ APR.

It is always worth checking the TAR (total amount repayable).

(And for the OP, you are taking into account Quidco cashback amounts, right? The third or fourth highest premium is often the cheapest overall.)