Despite what the Nokia Availability site says, the Cyan Update for Windows Phone 8.1 is out now on O2 at least.
Since early July I have been using the “Settings & Phone Update” option to check and this morning it paid off. After a small Nokia Update I was told my phone was ready for Windows Phone 8.1, then I checked again and it started.
I got the spinning cog wheels and am now exploring Windows Phone 8.1 for the first time.