Hi - It's a good question and one that a lot of people ponder; ourselves included. I suggest you ask your insurance agent that you have your auto policy with; we have done that and he said in most cases a well-insured policy holder in the U.S. doesn't need the extra coverage via the car rental company. For peace of mind, ask your agent about your specific policy and to provide scenarios of what would/would not be covered. Hope this helps!
By chance are you staying at any HGVC resorts in Scotland?
We are thinking of visiting Scotland for the first time ... We did spend almost 2 weeks in Ireland in 2010; lovely.
I have researched this issue to death. (You can see many of mine and other's posts under "Caribbean" (how it ended up there - I have no clue). We have decided to take out the car rental company's expensive insurance to the max. It's all in the fine print with the credit card company coverage- and that is diminution of value, fees and depreciation are not covered. This can be a big problem if you are in an accident. Also - liability for the driver. We are not taking ay chances. We had friends who got into an accident in Ireland and said they would never rent a car overseas again due to the out of pocket expense of that accident.(We do have an umbrella policy here at home that covers liability all over the world which is good to know - but the underlying coverage on the car rental insurance has to be 100,000/300,000, which I am assuming it is - but, who knows? Try to get a copy of it!) My husband actually works as an auto adjuster for a big company here in the states and he said this is even an issue in the US, but not as much so because you do have your personal policy in effect here, which helps. But the diminution of value can still be an issue, even here in the states (there is a post under Caribbean on this). (Look under trip advisor forums for overseas car rentals and you will see lots on this topic)
Anyhow - it's our first time overseas and we are staying at Kilconquhar Castle Resort in Fife. It's an RCI Gold Crown resort. we are renting from an owner. Flying into Edinburgh.