I honestly do not believe that longer sentences would act as a deterrent, the argument is the same for that of Capital Punishment, and the reasons for denying its efficacy are exactly the same.
Carrying a knife, like murdering someone, is not the action of a rational thinker.
Knives are not carried by people who think about consequences, who look ahead, who understand cause and effect.
I would willingly bet that every single convicted murderer would agree that they were not considering the consequences of their actions at the time they acted.
People who carry knives do so because they believe it accords them 'respect' from their peers, and that very thought process illustrates perfectly that they don't understand the first thing about the concept of what respect actually is, how it is obtained, and what it means in society.
People who carry and use knives don't think about consequences, responsibility, and certainly not respect - not what they call 'respect' which is actually just fear, and that's very different.
The idea of telling them that they will spend longer in jail merely encourages them to increase their 'reputation' with their contempt of the law, coupled with an inbuilt sense of invincibility that they obtain by mixing with like-minded isolated unhappy immature young people.
The way to stop knife crime is to educate children about respect for themselves and each other, and that is something the government fails to do, as have all others before them.
Treat the problem, prolong the problem, eradicate the cause of the problem, solve the problem.
inas