Cortana is one of the most highly anticipated features of the Windows Phone 8.1 update. Unfortunately, it's only available for users residing in the US, and will roll out to other regions in the coming months. UK is among the next batch of countries which will receive this new feature, but Windows Phone UK mentioned it will come later in 2014, not this month as previous reports hinted.
Recently, Marcus Ash, Group Program Manager for Cortana on Windows Phone said users rocking the developer preview in the UK will get Cortana in less than two weeks. This should come as a good news for all Windows Phone users residing in the UK.
Previously, Windows Phone UK Twitter account didn't drop an ETA, but it could come as an update to Lumia Cyan in the coming months. Lumia Cyan is rolling out in various parts of the globe, including the UK. Lumia 625 and 925 on Vodafone and O2 are receiving the update, but Cortana is not a part of it at the moment. It could also land as a separate update for UK users, but if you're rocking the developers preview, keep checking.
Thanks Anestis for the tip!!