[1] If you just want to use Cortana, you can do so by changing your region, speech and language to the USA. You'll get get US-Cortana.
[2] If you want to use UK-Cortana, download Preview for developers app and register with your Outlook account. Once done, go to Phone Update(You'll find in Setting) and check for updates. You'll get the required update - 1 for 8.1.
[3] Once done with installing update-1, you should change the language, Speech and region setting to UK and you'll get Cortana for UK.
08-06-2014 08:00 AM