Regardless of whether the emergency handle is connected to a physical brake or if it sounds a alarm, there is an emergency brake. If the driver removes their hand from the dead man's switch, the emergency brake engages. But that's a side-road to this discussion.
Unless you can point to official advice from TfL that states otherwise, I'm just gonna come out and say that I think it's best to stick to the advice TfL give at stations which is not to pull the emergency handle if someone is unwell. Whatever the precise action that results from pulling the handle (whether that's sounding an alarm, engaging a brake, or whatever), I don't see what it would achieve between stations and especially when TfL say not to do it.
Unless you can point to official advice from TfL that states otherwise, I'm just gonna come out and say that I think it's best to stick to the advice TfL give at stations which is not to pull the emergency handle if someone is unwell. Whatever the precise action that results from pulling the handle (whether that's sounding an alarm, engaging a brake, or whatever), I don't see what it would achieve between stations and especially when TfL say not to do it.