Regulation 44 defines what means of proof contracting authorities can require economic operators to provide as means to ensure their compliance with the technical specifications, award criteria or contract performance conditions.
The general rule is that contracting authorities may require the submission of certificates by economic operators. As with Regulation 43, even if the requirement is for the certificate of a specific certifying body (as defined by Regulation (EC) 765/2008), the contracting authority is under the obligation of accepting "equivalent" certificates. I assume that this is yet another situation whereby the Directive lawmakers feared a temptation by contracting authorities to use national certifying bodies.
Furthermore, again as with Regulation 43, an escape valve is provided to economic operators without the required certificates or test results: the contracting authority will be forced to accept alternative means of proof as long as the substantive requirements of the absent certificate are complied with *and* the lack of access to the certificate is not attributable to the economic operator. My take is that English and Welsh contracting authorities will not be able to exclude foreign economic operators for not providing certificates from the UK, but will be able to do so if the economic operators did not get an equivalent certificate from their own country. This assumes, of course, that the contracting authority knows that in Slovenia or Sweden there are equivalent certifying bodies and that the economic operator could have had access to the appropriate certificates on time.