I think he might take a more practical viewpoint, given that roughly 9% of the UK population use marijuana.
It's self-evident that sending all drug users to prison would reduce drug use, just by denying them access to supply. But it's simply not practical to do so. Our prisons are already overcrowded, so you'd have to divert quite a portion of the national economy into building, maintaining, and staffing prisons, and also suffer the removal of 9% of the population from the economy - currently, we have 0.001% incarceration rates.
So, given that incarcerating all drug offenders will essentially destroy the economy of the UK and turn us into an Island Stalag Luft, we have to ask the question, is there a less harmful way of dealing with the problems involved with drugs? Is it cheaper for the state to safely feed the habits of drug users, given the cost in social disruption and healthcare costs they cause through theft and the use of poor quality, unregulated product? Which solution causes the least amount of harm?
Whereas a politician stating that "Drug use is immoral and you should be locked up!" isn't providing any kind of justification. Pick one, he :
i) Hasn't thought it through. Should people who don't actually think about what they are saying actually be in charge of anything? Note that in this one I am including all the subspecies of not thinking things through, including religion, moral outrage, fear of a different culture, etc.
ii) HAS thought it through. Has a financial interest in the prison-industrial complex. Since 9% of the UK population is a drug offender, there is an effectively endless supply of "product" for the commercial prison system, since as we have already postulated, incarcerating all drugs users isn't actually possible given that it would probably plunge the country into revolution.
It makes far more sense to insist on locking up people who commit actual offences (other than just possessing drugs) to feed their drug habit. It makes even more sense to try and work out what causes drug habits and how to fix THAT.