The current direction on when an act can be classed as murder was handed down by the House of Lords in R. v. Woolin:
Where the charge is murder and in the rare cases where the simple direction is not enough, the jury should be directed that they are not entitled to infer the necessary intention, unless they feel sure that death or serious bodily harm was a virtual certainty (barring some unforeseen intervention) as a result of the defendant’s actions and that the defendant appreciated that such was the case, the decision being for the jury to decide on a consideration of all the evidence.
Now, this isn’t nice behaviour, to put it absurdly mildly. But it’s very clearly not something that would make death or serious bodily harm a virtual certainty. So murder it isn’t.
Manslaughter, on the other hand, is proved on the basis of ‘an unlawful act that is likely to do harm’… and I’m fairly sure I know what I’d say were I on the jury.