Update 6 June 2019 prof Andrew Dickinson reviews the case in L.Q.R. 2019, 135(Jul), 369-374. He concludes (references omitted) ‘the better argument is that the jurisdictional issue in Kaefer could and should have been disposed of straightforwardly on the basis that there was no evidence to demonstrate the existence of a consensus between the claimant, on the one hand, and the third and fourth defendants, on the other, that met the formal requirements set out in art.25. The written contractual documentation referred only to the first defendant as a party, and to the second defendant as a person to whom invoices were to be addressed, and did not refer to the third and fourth defendants at all. On this approach, English law, and its undisclosed principal rule in particular, had no role to play.’
 EWCA Civ 10 Kaefer Aislamientos v AMS Drilling et al is a good illustration of the difficulty of privity of contract (here: privity of choice of court), and the limits to the harmonisation of the rules on choice of court under Article 25 Brussels I Recast.
Herbert Smith Freehills have analysis of the wider issues of the case (over and above Article 25) here. The appeal considers among others the approach that courts should adopt when, as will usually be the case at the interim stage when a jurisdiction challenge is launched, the evidence before the Court is incomplete. Goldman Sachs v Novo Banco as well as Brownlie were referenced.
Appellant contends that the Court has jurisdiction to determine the claim against defendants AT1 and Ezion under Article 25 Brussels I Recast. It is said that the relevant contract contains an English exclusive jurisdiction clause and the relevant contract was concluded by AMS Mexico and/or AMS on behalf of AT1 and/or Ezion as undisclosed principals and, it follows, the contract, including its jurisdiction agreement, bound AT1 and Ezion.
At 81 Lord Green refers to the Privy Council in Bols  UKPC 45 which itself had referred to Colzani and Coreck Maritime (staple precedent at the CJEU; students of conflict of laws: time to worry if you read this around exam time and haven’t a clue). In Bols Lord Rodgers leading, held that CJEU precedent imposed on the court the duty of examining “whether the clause conferring jurisdiction upon it was in fact the subject of a consensus between the parties” and this had to be “clearly and precisely demonstrated“. The purpose of the provisions was, it was said, to ensure that the “consensus” between the parties was “in fact” established.
Lord Green (this is not part of the decision in Bols) adds that the Court of Justice has however recognised that the manner of this proof is essentially an issue for the national laws of the Member States, subject to an overriding duty to ensure that those laws are consistent with the aims and objectives of the Regulation. He does not cite CJEU precedent in support – but he is right: Article 25 contains essential, yet precious little bite in determining just how to establish such consensus. Prima facie complete, it leaves a vault of issues to be determined, starting with the element of ‘proof’ of consensus.
Of interest is that before deciding the issue, Lord Green notes at 85 Abela v Baardani  UKSC 44 (“Abela“) at paragraphs  and  per Lord Clarke and Lord Sumption, that to view permission to service out of jurisdiction as more often than not exorbitant, is unrealistic in the modern era: routinely where service out is authorised the defendant will have submitted contractually to the jurisdiction of the domestic courts (or there would be an argument to that effect) and in any event litigation between residents of different states is a normal incident of modern global business. As such the decision to permit service out is, today, more generally viewed as a pragmatic decision predicated upon the efficiency of the conduct of litigation.
It was eventually held that the evidence pointed against AT1 and Exion being undisclosed principals and that therefore the Court of Appeal was right in rejecting jurisdiction.
(Handbook of) EU private international law, 2nd ed. 2016, Chapter 2, Heading 2.2.9, Heading 126.96.36.199.