Welcome to FT, Socal Travel Junkie!
You've already been given some good advice, let me add a few things:
1. Check if your credit card covers car rental insurance. Most of the better ones have it and you can confidently say no to anything the counter tries to sell to you. This is what I do.
2. Check if your US car insurance policy includes foreign car rental, and at what level. Most good policies cover this.
3. Europcar is a very big rental company, around since the 1950s, owned by a French private equity firm (it used to be part of VW). Sixt is the largest rental company in Germany, Austria, etc, and was founded about 100 years ago and controlled by the Sixt family. They have the best fleet in many European family, including the UK.