> [IPP] was intended to protect the public against criminals whose crimes were not serious enough to merit a normal life sentence but who were regarded as too dangerous to be released when the term of their original sentence had expired. It is composed of a punitive "tariff" intended to be proportionate to the gravity of the crime committed, and an indeterminate period which commences after the expiry of the tariff and lasts until the Parole Board judges the prisoner no longer poses a risk to the public and is fit to be released. [1]
That last bit is even worse than it sounds: the burden of proof is on the prisoner to prove that they're no longer a danger to society, instead of requiring the Parole Board to prove that they're still a danger. Between that burden and the rest of the Parole Board's workload, it's taking forever to get anyone released.
IPP was eliminated in 2012 because it was unjust and overused, but politicians made the conscious decision to not retroactively undo it because of the bad PR it'd cause if some violent offender was released and killed someone.
[1] https://en.wikipedia.org/wiki/Imprisonment_for_public_protec...