The maximum sentence for GBH with intent is life imprisonment. The actual sentence passed (if your b/f is found guilty) will depend upon which of the three categories of the offence which are used for the purposes of sentencing. If the injuries sustained by the victim weren't at the very highest end of those which constitute GBH, it's likely that the 'kidnap' element will push the offence from the lowest category (3) into Category 2. If so, the minimum sentence that the judge can pass will be one of 5 years imprisonment, with a maximum of 9 years imprisonment.
If the injuries sustained by the victim were particularly serious, that (together with the 'kidnap' element) will push the offence into Category 1. That means a minimum sentence of 9 years and a maximum of 16 years.
Here's the actual document which the judge must refer to:
http://sentencingcoun...ine_-_Crown_Court.pdf
Chris