"We also accept that those who promoted the 2003 Act and Parliament that enacted it must have anticipated that the lifer regime …would be available to IPP prisoners so as to give them a fair chance of ceasing to be, and showing that they had ceased to be, dangerous. This was the context in which the legislation was enacted. To use Laws LJ's phrase[2008] 1All ER 138 , para 26, it was 'a premise of the legislation'"