In this guide, I will be showing you how to use Cortana on the Windows 10 Technical Preview in the UK.
Currently, Cortana is not available in the UK and the only way to start using it is to tell Windows that you are in the US. You can do so like this by going to the search menu and typing language.
You should see the option for Region and Language settings. You should select this.
Now select United States as the Country and add the English (United States language - Go to Add Language -> English -> United States)
Cortana should now work!