By law it is manslaughter. It wasn't premeditated.
It isn't premeditation that matters, it's intent to kill/cause GBH that differentiates the two. Premeditation is taken into account as an aggravating factor for sentencing and provides evidence for the jury, but you can still murder someone without having planned to in the weeks/days/hours/moments leading up to it.
I really struggle to understand how strangling someone isn't seen as intent to kill/cause serious harm though.