Similar to 'Joeysmummy' I now have JB!
I have the GS3 which I purchased from Carphone Warehouse as an upgrade to my O2 contract (O2's direct deal was worse!). It has an O2 contract sim in it obviously but I believe it is "sim-free"/"unlocked" as I purchased from non-carrier? It certainly has no O2 bloatware on it and does not startup with a O2 splash screen like my previous carrier-direct phones.
Yesterday I tried to update to JB via Kies and it said no update available.
Today after reading Joeysmummy's experience I connected (via cable) to Kies and the update was there!