England has a single-payer (the NHS) and a population of 53m people. So I guess the answer is England?

Nope. Medicare has 55.3M beneficiaries.[1]

[1]http://www.ncpssm.org/Medicare/MedicareFastFacts

57 million, but that oncludes 17.6 million that are in third-party at-risk plans where Medicare is just paying a portion of the premiums (Medicare Advantage), not traditional "single payer" Medicare, so don't count in Medicare's negotiating power. So there's just under 40 million in traditional Medicare.

http://kff.org/medicare/fact-sheet/medicare-advantage/

You need to be careful with that definition. For prescription drugs, Medicare just pays premiums to private insurers under Part D. Following your logic, Medicare has zero negotiating power for those drugs.

The NHS is for the whole of the UK, which is 64 million people.

The NHS is for the whole of the UK, which has a population of 64 million.

Technically there are four different NHS', one for each nation in Britain. Wales, Scotland and NI each report to their government rather than Westminster.

> England has a single-payer (the NHS) and a population of 53m people. So I guess the answer is England?

GP was being rhetorical, but if you really want to know, Medicare has 55 million beneficiaries in the US.

well none of them, since Medicare isn't a negotiator?

To extend GPs point, there are negotiators in the US who are bigger and therefore seemingly have more leverage than several of those countries. Therefore, perhaps it's not just a size/leverage thing.