Here is the bill ending release at the half way point for people convicted of terrorist offenders. There is a bold statement on the first page which I think may turn out to be wrong publications.parliament.uk/pa/bills/cbill… 
And here is the bit which I think makes the changes retrospective, and may lead to a declaration of incompatibility with Article 7 of the European Convention on Human Rights, the prohibition on retrospective punishment
This is why I think it could be a breach of human rights laws. Court will look at the intrinsic nature of the change - does it alter the *penalty* (if so, unlawful) or the *administration of the penalty* (if so, lawful)
UK courts haven't considered, as far as I can tell, if *automatic* release (by operation of law, no discretion to keep in prison) means the 'intrinsic' penalty is half of the determinate sentence, or whether the ability to recall on licence means the real penalty is full term
Nb. haven't properly considered bill, difficult to decode it as it is amendments to another act rather than a self-standing act, but I think point is people who would have been released half-way will now be eligible for parole at 2/3 but only if Parole Board says safe to release
