The prime test is that "there has to be an intent that the person to whom the threat has been issued would fear it would be carried out".
See the CPS website here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#15
and the wording of the legislation here:
http://www.statutelaw.gov.uk/content.aspx?LegT ype=All+Legislation&title=offences+against+the +person&searchEnacted=0&extentMatchOnly=0&conf ersPower=0&blanketAmendment=0&sortAlpha=0&TYPE =QS&PageNumber=1&NavFrom=0&parentActiveTextDoc Id=1043854&ActiveTextDocId=1043868&filesize=22 28
The law makes no reference to 'immediacy' and it would seem unreasonable for a court to require immediacy in order to convict. (e.g. If I told someone, with the intent that they'd believe it, "I'm going to kill you on Christmas Day", there would be no immediacy involved but it would seem unreasonable for a court to accept that as a defence).
The law also makes no reference to 'conditionality'. However, if the girlfriend, in your example, had already made her mind up that she was never going to return to her former boyfriend,there would be (from her point of view) no conditionality involved. (i.e. she knew she wasn't going to return and she'd now been told that she'd be killed for it). I doubt that 'conditionality' provides a universal defence but a skilled barrister might be able to persuade a court to give it consideration.
Chris