Often when UK residents book at UK websites for rentals in the US they get CDW/LDW (they are the same thing) and LIS thrown in for free basically. This is zero excess coverage. While the term doesn't really exist in the US as it relates to coverage, think of it as Super Cover. Bottom line, if you book at a UK site that includes the various coverages, there is no need to purchase anything additional.

I was not aware that UK residents get liability coverage with Amex Platinum, but LondonElite seems to indicate that is the case, so I will defer to him, but regardless, it would be redundant coverage if your deal includes LIS.