California would, in practice, need a variety of permissions from the feds to do approximately single-payer, which still wouldn't be actually single payer. Both the ACA and Medicaid would be major federal issues (in principal, it could opt out of Medicaid, but that would lose a lot of federal health care funding California is being taxed for). And it would still have the interface-to-other-systems problems single-payer is designed to avoid, because Medicare and VA would still exist and require coordination of benefits.

No European country would have similar problems, because they are independent nations.