According the Court's Press Summary :

There were two particular questions before the Supreme Court. First, was the detention of each respondent lawful, given that article 28 of the Regulation permits detention where there is a “significant risk of absconding”? The phrase “risk of absconding” is defined in article 2(n) of the Regulation as the existence of reasons in an individual case, based on objective criteria defined by law, to believe that the person might abscond. Secondly, if the detention was not lawful, are damages payable either under domestic law for false (or wrongful) imprisonment, or pursuant to what is known as the Factortame principle. 

Chapter 55 does not establish objective criteria for the assessment of whether an applicant for international protection who is subject to a Dublin III transfer procedure may abscond. Its contents do not constitute a framework with certain predetermined limits. Further, it does not set out the limits of the flexibility of the relevant authorities in assessing the circumstances of each case in a manner which is binding and known in advance. Therefore, the Court of Appeal was right to hold that Chapter 55 cannot satisfy the requirements of articles 28(2) and 2(n) of the Regulation. Chapter 55 does not satisfy the requirements laid down by the Court of Justice of the European Union in Policie ČR, Krajské ředitelství policie Ústeckého kraje, odbor cizinecké policie v Al Chodor (Case C-528/15) Because Chapter 55 does not set out the limits of the flexibility of the relevant authorities in assessing the circumstances of each case in a manner which is binding and known in advance, it lacks the necessary qualities of certainty and predictability. It therefore does not constitute a “law” for the purposes of articles 28(2) and 2(n). A broader question is whether a statement of policy and public law adherence to it can ever amount to a binding provision of general application and so a “law” within the meaning of article 2(n). That question should be decided in a case in which it is necessary to do so.