Cortana Developer Preview comes to the U.K. for Windows Phone 8.1 in less than two weeks2
Right now, only those in the U.S. with Windows Phone 8.1 are able to use Cortana. In his conversations on Twitter, program manager Ash said that while developing Cortana, Microsoft learned a lot about scaling; that is certainly one of the issues that the company would have to face when rolling out a project as ambitious as a virtual personal manager, to a large number of countries world-wide.
No word from Microsoft yet, as to when non-developers in the U.K. will be able to access Cortana.
source: @marcusash via GSMArena