Why are NHS Commissioners 'undertakings' and, consequently, subject to competition law?

Some months ago, I held an interesting email exchange with some readers of my paper New Rules For Health Care Procurement in the UK. A Critical Assessment from the Perspective of EU Economic Law. They basically challenged my understanding of the National Health Service (Procurement, Patient Choice and Competition) (No. 2) Regulations 2013 on the basis that NHS Commissioners could not be considered undertakings and, hence, their decisions should remain outside the scope of application of competition rules.
However, I thought and still think that NHS Commissioners are 'undertakings' for the purposes of (EU) competition law enforcement. These are the main reasons why I think so (apologies to non-competition law readers for the amount of 'slang' in this post, which reproduces parts of the email exchange.
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Regarding the treatment of NHS commissioners as undertakings, I think that the FENIN/Selex exemption is inapplicable and probably I should have made this clear in my paper (I simply assumed that this would not be controversial). As you probably know better that myself, the reason for that is basically that (most) GPs are engaged in economic activity as self-employed providers of services to the NHS (http://www.nhscareers.nhs.uk/explore-by-career/doctors/pay-for-doctors/) under the so-called General Medical Services Contract (http://www.nhsemployers.org/PayAndContracts/GeneralMedicalServicesContract/Pages/Contract.aspx). Indeed, they hold contracts for the provision of those services and, consequently, everything that they procure or commission needs to be assessed in light of such ‘downstream’ or parallel economic activity (which, in my view, immediately deactivates the FENIN/Selex exemption). Moreover, GPs located in a given area are in competition between themselves in order to attract patients and retain them, and that has an impact on their level of remuneration by the NHS. All this indicates that they do engage in economic activity ‘downstream’ or in parallel to the services and goods that they commission and purchase in their public procurement (‘upstream’?) activities. That is enough to justify the direct applicability of competition law (EU and domestic) to their activities.
In my view, this conclusion is robust even if those services are generally not directly paid for by the end users in most of the cases, since that should not affect either: 1) their inclusion within the scope of application of EU internal market law (C-372/04 Watts, dealing particularly with the NHS, although with hospital care provision), or 2) the fact that GPs are undertakings, as the requirement of provision of services in the market for remuneration does not require direct payments; under the classic formulation of the concept of an undertaking, it encompasses every entity engaged in an economic activity, regardless of the legal status of the entity and the way in which it is financed. So, I guess that the largest point of disagreement between us is that you may consider that GPs (individually or collectively as part of a Clinical Commissioning Group, CCG) are not engaged in economic activity. However, as self-employed providers of services, I think that that assessment would not be in line with the generally functional approach to the concept of undertaking and that it is not covered by the FENIN/Selex case law. I do not think that GPs would be covered by the ‘social’ exclusion for systems based on solidarity either, given that the system in the UK promotes choice and competition and, by itself, that goes against the requirements of mandatory participation that the CJEU has included in its sickness funds-relate case law.
Finally, I also think that there would be a possibility of circumventing any possible exclusion of the (direct) applicability of the rules to the GPs and CCGs as undertakings (or groupings/associations of undertakings) via a State action doctrine approach (basically, on the basis of Cipolla) given that the UK as a State has delegated economic decisions on a type of organs (CCGs) that are in a structural (mild?) conflict of interest when they adopt commissioning (economic) decisions and, consequently, liability (of the UK) could be found on the basis of Art 4(3) + 101 TFEU. That would clearly justify the consideration and application of EU competition rules by Monitor as the ultimate watchdog in charge of ensuring compliance with (EU and UK) competition rules—as it is indeed co-competent with the new CMA in the healthcare sector. Again, you may consider this a weak legal basis, but I would disagree with that.