@evleaks leaked an image from Nokia about the release date schedule for Windows 8.1 (
Windows Phone 8.1 Release Schedule for Nokia Lumia Devices Revealed via Leaked Roadmap).
This shows as late June/early July for most devices. I would imagine taking into account the tweet from @marcusash saying that 'Making #Cortana available in the UK and China will happen very soon. Weeks not months.'
https://twitter.com/marcusash/status/477959992650117120 (June 15th) I would guess the two will come near enough hand in hand (usually Nokia release there updates for most regions at once).
With the guess work above, I'd predict that HTC wouldn't be too far behind Nokia with 8.1/Cortana so I would think that we can all look forward to Cortana in the UK on the stable update channel very soon!
I might be wrong though, just trying to make a square fit through a triangle hole!