Don't they also have private healthcare insurance in Canada, Germany, and England?
All those systems are different than the Sanders M4A policy and from each other. Also, the Sanders plan was basically going to ban most private coverage for things the government would cover.
If I recall:
Canada's system doesn't allow private coverage of items covered by the public plan. Prescription drug coverage varies by province, and frequently requires supplemental private insurance. However, drug costs are kept lower by government involvement in pricing.
The UK has a single-payer and state run healthcare delivery system. But there are also private alternatives available.
Germany had a system much like an ACA on steroids. I believe most of the private insurers are not-for-profit entities. Again, prices are kept down by heavy government involvement.