Kidnapping is an offence against common law. i.e. there isn't a specific statute passed by parliament that makes it a crime, it exists as a crime by precedence because it was recognised as a crime by the courts long, long ago before the criminal justice system was anything like it is today and that has been considered sufficient. Murder is the same.
There are no sentence boundaries attached to it so judges are able to set any sentence, including life. But they should be guided, normally by the sentencing Council, but they don't seem to have any guidance on it so, in lieu of that, as before the Sentencing Council and its predecessor, by the pronouncements of higher courts and by precedence set by case law.
From <a class="break-all" href="https://web.archive.org/web/20120421181525/www.cps.gov.uk/legal/s_to_u/sentencing_manual/kidnapping_false_imprisonment/" rel="nofollow" target="_blank">an archived CPS page, here's the guidance that used to apply:
Guideline case R v Spence and Thomas (1983) 5 Cr.App.R.(S.) 413
There is a wide possible variation in seriousness between one instance of kidnapping and another. At the top of the scale comes the carefully planned abductions where the victim is used as a hostage or where ransom money is demanded. Such offences will seldom be met with less than 8 years' imprisonment or thereabouts. Where violence or firearms are used, there are other exacerbating features such as detention of the victim over a long period of time, then the proper sentence will be very much longer than that. At the other end of the scale are those offences which can perhaps scarcely be classed as kidnapping at all. They very often rise as a sequel to family tiffs or lovers' disputes, and seldom require anything more than 18 months' imprisonment, and sometimes a great deal less.
and (from the same page), quoted relevant case law:
R v Dzokamshure [2009] 1 Cr. App. R. (S.) 112
Appellant's relationship with a woman for six months had ended. Months later he went to her home, broke his way in, punched her, dragged her out and forced her into her car. Another man was seated therein. The appellant drove the car on the motorway and prevented her from answering a call on her mobile phone. Eventually he stopped on a slip road and allowed her to get out. Previous good character. Guilty plea. The victim had indicated that that she did not want him to serve a custodial sentence and was unwilling to give evidence. Sentence of 18 months' imprisonment upheld.
[My emphasis both times]
That last case isn't from all that long ago. I think over the last decade or so the Government and the courts have begun to take domestic violence far more seriously. (Maybe this is why the Sentencing Council have failed to come up with anything so far because they can't decide between erring towards precedence or more contemporary pressures without more guidance from government? I don't know. Maybe it's another reason like they have to be asked to cover common law crimes). In any case, given that guidance and what has been the case in the past, I think the judges sentence is wrong, but unsurprising. I hope the court of appeal make the sentence much longer and that sets precedence for the future.