The judge can never direct a jury to convict, but the Crime and Disorder Act 1988 gives them the power to dismiss a charge if they conclude that there is not enough evidence for them to be properly convicted. Whilst the CPS should only go to court when they think there is enough evidence to give a reasonable prospect of conviction, sometimes they misjudge it or, more commonly, their case falls apart in court. If there really isn't enough evidence for a conviction to be safe, why risk the jury getting it wrong?
The judge directs the jury as to the law and other relevant factors they need to take into account when considering the evidence (e.g. the need to treat identification evidence with caution). It is also becoming increasingly common for the judge to provide the jury with a flowchart or similar aid, setting out the legal and factual issues they have to decide and what verdict their decisions lead to. This does not tell the jury what their decisions must be, it just helps to guide them. To take a simple example, in a rape case the guidance might be something like:
Did the defendant have penetrative sex with the complainant? If the answer is no, your verdict must be not guilty. If the answer is yes, move on to the next question.
Did the complainant consent to sex with the defendant? If the answer is yes, your verdict must be not guilty. If the answer is yes, move on to the next question.
And so on.
The judge cannot, under any circumstances, make any findings of fact, so they cannot decide whether the prosecution witnesses or the defence witnesses are telling the truth. That is entirely for the jury.