Are you editing after the fact? I don't remember a lot of that text being there a few posts ago. It just might be my insomnia as well...
You're right iraq pulled out of the ICC at the behest of the US a few years ago. There have been documented cases of UN security council members requesting investigations and or a tribunals which have been granted. In those cases the Hague, being founded on both the Hague and Geneva articles (to which Iraq is still a signatory), becomes the court of choice. I am not so sure whether the power of veto is available in these circumstances. In any event it is feasable, but being the UN it would be a nightmare to even attempt. Getting UN members to play nice is an impossible task.
Since the UK courts are handling this, my guess is it has fallen under the purview of private international law to which both are signators of Hague/Geneva. That commonality could transfer jurisdiction if both agreed. I suspect the real motive is that if these guys have to serve time, then they'd rather do it in the relatively safer UK penal system.