OK, here is what I have learned and what I decided to do!
First, I went onto Rick Steves' website and read about travel insurance:
https://www.ricksteves.com/travel-tips/ ... -insurance. Then I did some price comparisons using the items in the little box on the right, but they were all expensive and set up for people who were purchasing group tours and cruises. I have been making my own plans and really do not have that much invested so far, other than my airfare and a few hotels that required deposits--no more than $2,500 at this point. Mostly, I am concerned about medical problems abroad, like breaking a hip or being in a car accident, etc.
Next, I called United Healthcare. I have a Medicare C plan through them. It is an "F" plan, which is the most expensive, but DOES have $50,000 worth of lifetime medical expenses incurred abroad. Only problem is that you have to pay first and get reimbursed later. Also, it does not cover things like medical evacuation back to the U.S. if say you broke a hip or had a major accident.
So, I did more searching on the little box on the Rick Steves' page and contacted InsureMyTrip at
https://www.insuremytrip.com/. They compare and sell insurance from many companies, so they can do a super job of comparison and getting you exactly what you want. I still wasn't finding what I wanted so I called them and explained my situation. An extremely nice lady found me a medical only policy that gave me an extra $50,000 of coverage and a million $$ of medical evacuation coverage for only $85. It also had some dental, lost & delayed luggage, and travel delay coverage.
-- I will still have to pay first and get reimbursed later, but it is highly unlikely that anyone will hold me hostage in the UK until I pay my bill. (Unlike Mexico and some S. American or other places.)
-- It is VERY important to buy travel insurance within 15 days of putting your first deposit on something. For me that was way back last September, but I have been lazy. If you do that, they will waive any pre-existing condition policies.
So buy your insurance as soon as you buy the airline ticket or put a deposit on anything. -- My United Healthcare policy is still primary and the new policy will cover only after I submit to them first. But in a really serious situation having that extra 50,000 will really be nice.
Anyway, it was really nice to talk to someone who could explain what all the coverage really meant, so I really recommend these people.