Well, well, well. Baldric's link is fascinating - I once had insurance with a company which AXA swallowed up. I had renewed it for more than 20 years, including a few after the name change, and then finally I had a claim. AXA eventually paid up but they put me through a humiliating process to get the claim honoured - I swore never to take out insurance with them or any company underwritten by AXA. I have kept my vow ever since, and that was at least a decade ago and Baldric's link gives me a warm feeling as confirmation that my decision was correct.
Car excess insurance relies on you providing the documentation from the car hire company - yes, you pay up front and then claim it back from your insurer according to the receipt(s) you have. I have had this sort of insurance (annual policy) for many years now - happily I have never yet had to claim but the peace of mind is worth it although I suppose I will only find out if it works once I make a claim. The same insurance cover sold by the car hire company is horrendously expensive (that is the way they make money). The company I have mostly used are insurance4carhire.com although currently I am insured with icarhireinsurance.com, both operate on the same principle (untested). There are of course others. If you want to reverse your purchase you should be able to so long as the cover period has not started, I imagine you should have a good chance of a full refund.
One word of advice, even if there is no damage to a rented car when you return it, make certain you get from the hire company a written statement to say there is nothing further to pay, ideally with a note saying "Nil" for damage/"other". Keep that for at least six months (or as long as your card issuer advises) - there are a few stories going around to the effect that an after-the-event charge for alleged damage has been made to the renter's card account. It doesn't matter whom you are renting from, it is simply good policy to have the documentation.