Making the punishment fit the crime is only one aspect of the problem. Another aspect is that the punishment should make the criminal think twice before reoffending !!! The victms of crime must also be reassured that the law is there to protect them.
Generally judges have sufficient experience to know whether an offender can safely be given a second chance, or not. Guidelines, written or not, curtailing judges sentencing powers, are not the ideal method for achieving "Crime reduction". If there is not enough prison space available to house criminals, then more prisons must be built.
Stiffer sentences are not always the answer either, but doubling the sentence each time for reoffenders, would in my mind make criminals think twice, including petty offenders. This would free-up prison space.
David Cameron senses that laws must be passed to attain sane objectives. Politicians should not flinch nor be frightened !!! The electorate has already had enough of the effect of soft justice. Political "do gooders" who advocate soft justice are the ones who will lose votes !!!