Health - set up an MHS (Marijuana Health Service) to run in parallel with the NHS, distribute seeds at minimal cost to anyone that wants to grow their own medicine at home, advice given on which strains to grow for their particular illnesses on a free government run website along with growing advice, surplus medicine to be sold back to the MHS at a reasonable price, depending on quality and demand for that particular medicine. Medicinal cannabis grows also set up and operated by the MHS and people encouraged to grow their own medicine whilst providing a facility to sell their surplus to the MHS. Encourage habitual drinkers to switch to cannabis, this alone would save the NHS a fortune. For certain illnesses I would make the new MHS, staffed by cannabis specialists, the first port of call.
Prisons - The average reading age of prisoners in the UK is around that of an 11 year old, I can't imagine how difficult a normal working life must be with that sort of handicap - so massively increase education in prisons to address the current, hideous, recidivism rate.
Out of 80,000ish prisoners currently incarcerated in the UK less than 100 are considered a danger to the public, so I'd implement a scheme where employers are encouraged financially to employ ex-cons, who are on a tag and under curfew and incentivised by a conditional significant sentence reduction.
Energy/Environment - As Henry Ford initially intended, build cars from hemp - significantly lighter than steel, significantly stronger than steel and much, much better for the environment than steel in terms of both production and running.
Set up new anaerobic digestion plants and run them all on hemp not maize as they currently are, this would localise renewable electricity and gas supplies.
I'd set up a 'Hemp Commission' to work closely with the MHS on supply issues, with the remit to explore which of the 25,000 uses hemp has brings the most benefits, they would also be tasked with ensuring supply by maximising the growing space available in the UK and potentially partnering with the closest West African country most suitable for growing hemp in terms of cost/access etc.
Drugs - legalise and regulate them in the same way we do alcohol, get the police force back to investigating burglaries, rape, assault, fraud etc and re-building relationships with the public.
It's beyond ridiculous to have the most dangerous drug legal and celebrated whilst less dangerous drugs are prohibited.
Education - at around 11/12 use specialist computer software to explore which career path is likely to be most suited to each student and tailor their education accordingly as far as possible, if it's something practical allow them to spend time on a part time basis in that environment starting with a day and progressing to full employment when considered suitable.
I'd also incentivise older students to do some informal in school teaching on a voluntary basis, the best way to find out how much (or little!) you know is by teaching it.
For clarity, hemp and marijuana are both cannabis, the only difference is hemp contains less than 0.3% THC and doesn't get you 'high'.