So if a government funded, administered and managed health care service which is  all encompassing and 'right' for all citizens why is there a thriving market for supplemental insurance plans in both the UK and Canada, two countries often touted as being the best solutions for the US to follow?

Additionally to compare public roads with the complexities of healthcare is not an argument that holds much validity. But if we examine the public road argument you would see that very few if any roads that were actually built by government entities. Often those jobs are put out to bid to private companies, thus lowering the costs for tax payers. 

Most members in the GOP have not embraced a fully free market however. In fact many still hold in high regard Medicare and Medicaid. Both programs which are beset with issues and problems too numerous to document here. Of course one of the largest proponents of Medicare and ACA , AARP love the programs, why? They make huge money filling in the holes.