Reg.25 of the Public Contracts Regulations 2015 (PCR2015) follows closely the drafting of Art 25 of Directive 2014/24 and imposes the duty of equal treatment (ie treatment no less favourable than than accorded to EU operators) to the works, supplies, services and economic operators of the signatories of the World Trade Organisation's Government Procurement Agreement (GPA), and other international agreements by which the EU is bound.
The only difference between Art 25 Dir 2014/24 and reg.25 PCR2015 is that the former refers to "annexes 1, 2, 4 and 5 and the General Notes to the European Union’s Appendix I to the GPA", whereas the UK's domestic rule extends the non-discrimination obligation to "annexes 1, 2 and 4 to 7 and the General Notes to the EU’s Appendix 1 to the GPA". Hence, there seems to be a discrepancy between both rules, and the PCR2015 seem to include more parts of the GPA's annexes than Dir 2014/24. However, on closer look, it is clear (?) that there is no such difference on substance.
Understanding why there is no actual difference requires some digging on the scope of coverage of the GPA and its very recent modification, which entered into force in April 2014 (ie after dir 2014/24 was adopted). In
the 1994 version of the GPA, the coverage was structured in 5 annexes
plus general notes (*). In the revised 2011 version (effective 2014),
the coverage was reorganised in 7 annexes, the last being the general
notes themselves (**). Both annex structures however have in common that Annex 3 deals with
the coverage of "other entities".
2014/24 refers to the 1994 version of the GPA (still in force when it was
approved), whereas the PCR2015 refer to the
2011 GPA [see definition in reg.2(1) PCR2015, according to which “GPA” means the Agreement on Government Procurement between certain parties to the World Trade Organisation signed in Marrakesh on 15th April 1994 as amended--with reference to Council Decision 2014/115/EU on the conclusion of the Protocol Amending the Agreement on Government Procurement]. Hence, the drafting difference derives from the modification of the GPA between the approval of Dir 2014/24 and the PCR2015. Ultimately, though, reference to "annexes 1, 2, 4 and 5 and the General Notes to the European Union’s Appendix I to the  GPA" in Art 25 Dir 2014/24 and to "annexes 1, 2 and 4 to 7 and the General Notes to the EU’s Appendix 1 to the  GPA" in reg.25 PCR2015 means the same: "everything but Annex 3 on other entities".
Regardless of this drafting complication, there is not much more to say regarding the transposition of this provision. The more interesting outlook refers to the eventual completion of the European Commission's project on restricting access to public procurement by third country undertakings (ie non-EU, non-GPA, not-covered by other treaties), which seems to be gaining traction after some hiatus [for background discussion, see K Dawar, The "Proposed ‘Buy European’ Procurement Regulation: An Analysis" (2012) CEPR Global Trade Alert 89-98]. However, that exceeds the purpose of this comment.
For Pedro's interesting complementary views, mainly linked to the TTIP, see here.
(*) Annex 1: central government entities; Annex 2: sub-central government entities; Annex 3: other entities; Annex 4: services; and Annex 5: construction services. These were accompanied by unnumbered general notes.
(**) Annex 1: central government entities; Annex 2: sub-central government entities; Annex 3: other entities; Annex 4: goods; Annex 5: services; Annex 6: construction services; and Annex 7: general notes. This makes the reference to both annex 7 and general notes an unnecessary repetition.