I've had some awesomely crappy experiences with both rental car aggregators and third party rental insurances. The combination is particularly toxic: the rental aggregator provides an insurance which requires you to shuffle papers between the aggregator and the actual hire company, leaving you to pay out if the papers are not shuffled to the aggregator's satisfaction. Why put yourself in that situation when you can avoid it by paying the hire company's insurance?

Meanwhile, prepaid rental vouchers leave you unable to give the hire company the finger when you mess them around. Summary of actual exchange at Dollar at Kona airport in Hawaii:

"Hi, I pre-paid for an SUV."
"We have this nice convertible for you."
"I ordered an SUV. Can I have that SUV there?"
"Only if you pay a lot extra. We only have the luxury SUV left. Or you can take this convertible."
... since I had need for an SUV, I had to pay extra. If I hadn't prepaid, I would have gone to another rental company, any other.

BA might have some reputation to keep and might have a decent arrangement with Avis, but all the online "cheap car rental..." places care not one whit for customer service.

So, my advice: Go direct to the hire company and don't prepay so you can walk away if they mess you about. If you do choose an intermediate agent, choose one with less bad customer service. Take all the insurances with the hire company, not separately. Eat the costs as being the cost of a quiet, unstressed life.