Contrary to popular belief ISAs are not completely tax free as explained in the income tax section of this site.
Inheritance Tax is another of the major taxes to which an ISA is not exempt.
Upon death any savings or investments held within an ISA are returned to your Estate and lose all forms of protection offered by an ISA.
As you plan for the end it may become good advice to start phasing out of ISAs as part of an estate planning process if you expect your estate to be worth morte than your IHT allowance., if you think this could possibly apply to you then you really should take professional advice from an adviser such as myself.
Levels, bases and relief's from taxation are subject to change.