Firstly, the report doesn't even cover the entire developed world, just 11 countries. Notably, Japan doesn't appear to be included. To be honest, I found that slightly offensive.
Secondly the U.K. isn't the overall clear "best" of those 11. It's not bad, and does better on some things, worse on others.
In my subjective experience (as a Brit) British people often think the NHS is the best health service in the world. It's not bad, and it's free. Waiting times are often long, particularly for non-urgent conditions. Compared to the U.S. system I prefer the NHS, but there are other options, and the NHS could provide a better service than it does.