Reg.81 of the Public Contracts Regulations 2015 (PCR2015) transposes Art 81 of Directive 2014/24 and imposes two straightforward and minimal rules controlling the composition of the jury for design contests. See Pedro's remarks here.
Firstly, it is clear that the jury shall be composed exclusively of natural persons who are independent of participants in the contest [reg.81(1) PCR2015]. Secondly, where a particular professional qualification is required from participants in a contest, at least a third of the members of the jury shall have that qualification or an equivalent qualification [reg.81(2) PCR2015].
Regarding the first requirement of independence of the members of the jury, it seems quite natural that the absence of conflicts of interest will need to meet the requirements of reg.24 PCR2015 and that, in the case of unavoidable/unavoided conflicts of interest, the conflicted participant should be excluded from the design contest as per reg.57(8)(e) PCR2015 (although, once more, the fact that such ground for exclusion is discretionary does not really help much).
Regarding the second requirement, from a technical perspective, it makes sense to require that a significant part of the jury holds qualifications needed to assess the project from a technical point of view. Of course, the requirement for participants and members of the jury to hold such qualification needs to be justified and proportionate. And the general rules of the Services Directive and, generally, on professional regulation and free movement need to be respected.
Those issues are not specific to design contests and would require comments beyond the scope of our procurement tennis. Suffice it to raise here that unreasonable or disproportionate requirements regarding (unjustified) professional restrictions could fall afoul of Art 101 TFEU as either a concerted practice or a recommendation of an association of undertakings [for an introduction to this general discussion, see IE Wendt, EU Competition Law and Liberal Professions: an Uneasy Relationship?, Nijhoff Studies in European Union Law vol. 2 (Leiden, Martinus Nijhoff, 2013)]. Hence, contracting authorities will be well advised to seek competition clearance where there are risks of illegitimate or unjustified foreclosure of non-qualified participants.