I think it will ultimately be the money left on the table that will make the UK government finally move, because the idea of taking an illegal market that is worth billions and taxing it is incredibly enticing, even for right wing politicians. Several of them have invested in North American cannabis operations already.
But public opinion must be undeniable for tory governments to act. Labour, Plaid and SNP can easily make the decision without offending their voter base, but tories can't. Millions of old people still around that grew up with 'All drugs are bad' ingrained into them from childhood, so have never experienced them to have an independent opinion of. Like old racists, their views cannot be changed so easily, and tories count on their votes heavily for power.