Cortana isn't going to work since the WWE firmware (basically all unbranded phones) are shipped without US english phone language support.
The ****ty thing is that it is the same language (there is no difference between the US english and UK english) but it just isn't labeled as such and thus wont support cortana. Considering the (lack of) support from HTC, this is never going to happen. (you can check this, change the phone language from UK english to any of the other english phone languages, it won't even have to reboot).
All of you with US branded 8x' are in luck today. Too bad for the ones that unbranded it.