The police and CPS are acting correctly by not taking into account any element of provocation. An illegal action remains a criminal offence however great the element of provocation might be. However, the court can take such matters into account when determining the sentence.
Although the court will impose separate sentences for the assault and the false imprisonment, the court will probably be more inclined to think of the offender's actions as a single composite offence. So it's the total sentence, rather than the individual components of it, which is relevant.
You've not indicated the degree of assault involved. (Common assault? ABH? GBH?). That will obviously have a considerable bearing upon the sentence. There are also other factors which are relevant. For example, leaving someone on the moors, who's still in a fit state to walk, just a few miles from help is not the same as incapacitating them and leaving them 20 miles away help on a freezing cold night. (The former action does little more than to inconvenience the victim; the latter is akin to attempted murder).
Normally, a 17-year-old first time offender would be given a referral order:
http://www.yjb.gov.uk/en-gb/yjs/SentencesOrder sandAgreements/ReferralOrder/
However, it's likely that the court would consider that the offences are too serious for that course of action in this case. If so, a custodial sentence is inevitable. If the assault was 'common assault' or the lower end of 'ABH', it possible that he might only be sentenced to 12 months youth custody. However, it's probably still more likely that the solicitor's estimate of 2 years is accurate.
Chris