Those countries, especially the UK, have issues with their systems.  For instance, in the UK, there are limits on where you go for general care providers.  Also, there is much more rationing for care in many countries with single payer.  So it may help people at the bottom, but it would harm lots of middle class people who now have access to very good health care in the US.  

And I am fine with the Democrats expanding the public parts of the US health care system to get more people covered.  This is what we all pay taxes for.  If Obama wanted to, he could have just gotten rid of some of the Bush tax cuts and used it to expand Medicaid.  I'd be cool with that as it would affect the top 1%, not the middle class.  By contrast, the ACA  and single-payer affect the middle class while the 1% can still pay for good care out of pocket.