Check your travel insurance carefully, and speak to the company, tell them what you are planning to do.

I think you will find that it has a box to tick if you will be driving a lease / rental car. If that is the case, you will probably find that the CDW and supplementary liability are already covered. Absolutely no point in paying for the same insurance twice.

From my experience rental companies have just taken a photo copy of my insurance policy, and not charged insurance.

Lifey who is confident you would not dare travel without insurance