Verdict directed by judge

A judge can only ever direct an acquittal. He can do this where, e.g., the prosecution has failed to present evidence proving an essential element of the charge. If there simply isn't evidence on which the jury can properly convict, he can tell them that they must acquit.

He can't ever give them an unqualified direction to convict. He can say something like "if you are satisified beyond reasonable doubt as to X facts, then you must convict". But it's still for the jury, not the judge, to decide whether the evidence produced does satisfy them as to X facts, so they can acquit.

What if the jury is satisfied as to X facts, but they want to acquit anyway? Suppose that the evidence proves beyond reasonable doubt that the defendant was in possession of cannabis, but they want to acquit because they don't think it should be a crime to posses cannabis?

As a matter of practical reality, they can acquit. This is, legally speaking, improper; they take an oath to apply the law, and they are refusing to apply the law, so they are not doing what a jury should do. But there is no sanction or penalty for this because they don't give reasons for their verdict. They just say "we find the defendant not guilty" and, for all anyone knows, this may be because they weren't persuaded by the evidence that he definitely did have the spliff. And there's no appeal against an acquittal in these circumstances, so the defendant will walk free. But it isn't a legal right of the jury to refuse to apply a law that they find unconscionable; it just a matter of practical reality that, as the system is set up, if they do refuse to apply it there is no mechanism for correcting that.