The answer to my own question posed in an earlier tweet is that the U.K. cannot derogate from article 7 the prohibition on retrospective punishment. Eg this case! echr.coe.int/Documents/FS_D… 
The Spanish case is interesting! The European Court of Human Rights very focussed on the sometimes blurry line between the penalty (sentence) and the “Manner of execution”. Focus was what it *actually entailed* in domestic law hudoc.echr.coe.int/app/conversion…
What is the “intrinsic nature” of a sentence with an “automatic release date”, ie one where release is by automatic operation of law rather than a discretionary decision by eg the parole board? Possible to argue the ‘intrinsic nature’ is custody ends at halfway point
That is of course subject to good behaviour as rest of sentence will be under parole conditions eg of good behaviour. But the ‘intrinsic nature’ of the sentence is automatic custody until half way point then automatic release for rest subject to parole
The law changes announced tonight arguably intrinsically alter the ‘intrinsic nature’ of the sentence as there will no longer be automatic release at half way point and indeed there is no prospect of release until 2/3.
Counter argument is that the *penalty* (the full sentence) remains the same. Lots of case law about imprisonment being automatically lawful of order of court permits it as it does during criminal sentence. Interesting!
Some lawyers have pointed to two cases - Uttley and Hogben. But both were about changed to the date when *discretionary* release possible which is different to change to *automatic* release by operation of law
