This kind of sentence no longer exists. However, as for life sentences, the minimum tariff is set by the judge during sentencing. It was not challenged because an 8 year tariff represents a 16 year sentence (an offender sentenced to more than 2 years will normally serve half their sentence in prison and the rest on licence in the community) and, given that he was only convicted of one rape, some sexual assaults and some drugging offences, it was not unduly lenient against the sentencing guidelines as they stood at the time.
If he had been convicted of anything like the number of rapes he is alleged to have committed the tariff would have been much higher.
I note that Theresa May knows one of his victims and that she is talking about reviewing the system in light of this case to bring greater openness. That, at least, is potentially one good thing to come out of this case.
By the way, I agree with MrsHathaway - "I would support a campaign that asked the authorities to prosecute him for the outstanding charges, to support his (currently legally alleged) victims during the new court process, etc." The big question, to which I don't know the answer, is whether there is actually enough evidence to convict him of further rapes.