To certain degrees they are free to make any judgement they so wish, in extreme cases, the Judge has the right to over-rule, but that often leads to a re-trial rather than a change of judgement, they very rarely do that however.
At the conclusion of a UK trial, the Judge summarises the case for the Jury, and lays out the facts they are to consider, often it is there that the Judge will mention not what results are expected, but rather the points of law that they need to take into account, and what evidence is 'solid' for both sides, so the Prosecution mentioning that the accused is, for example, a Heroin addict might be considered a possible motive in some cases, whereas it may have no bearing on others. The Judge will, to certain degrees, try to clarify what the law expects of the Jury with regards to the decision process rather than the verdict itself.