We did a breakaway in England prior to our GCT tour and rented a car from Hertz so I did some checking on insurance. If you reserve through Expedia they sell auto insurance with a $200 deductable for very good prices about $33 for a 4 day rental. The option I went with was using my Navy Federal Credit Union VISA Signature card to pay for the rental. VISA had no deductable and cost nothing extra.
I ended up using the insurance after having a close encounter with a Kentish wall. They were doing construction on the main road and diverted (There are no Detours in England only Diversions) all traffic down a very narrow country road. We did ok until there was a brick wall sticking out into the road which broke the left hand mirror and scraped the left side of the car adding to the scrapes that were there when we rented the car. We stopped just past the wall and the owner of the house we stopped in front of came over to help us out. He got some duck tape to tape the broken mirror and told us that people hit that wall all the time and we should complain to the highway department. All the people we met in southern England were very friendly and helpful. I highly recommend visiting the area while in England, since it is not included on the GCT tour.
When we returned the car Hertz noted the new damage and then used their computer system to calculate the cost of the repairs. I charged the repair and rental cost to my VISA and they printed out a detailed list of the damage and cost to repair. Upon returning home I contacted VISA and they sent me a claim form. I submitted my claim along with a copy of the rental agreement, a copy of my VISA statement showing the rental charge, and the list of repairs that Hertz provided. The insurance company that handles claims for VISA then emailed back asking me asking for a bill of repairs showing the type and cost of repairs. I decided to then call them to ask what more they needed than what Hertz had given me and after pointing out to the represented where on the Hertz document it listed the detailed repairs he thanked me and said payment would be sent soon. I got a check for the full amount a few days later.
All in all I was very pleased with how VISA and Hertz handled the process. The only thing I would have done differently was to take a picture of the damage to the car. I took a picture of the damage to the car when we rented it but not the damage on return. They requested a picture, but it was not required.