As far as I knew, FoV was available as a download for all versions of ED on the PC (even the Steam one, although you had to buy FoV outside of Steam). That's fine - after all the Dragon Age add-ons also had to be purchased outside of Steam.
I wasn't sure of this FOV digital download bought outside of Steam would work with Steam ED. I searched online for a while and basically came to the conclusion UK users couldn't buy it. So I just bought DKS on Steam with my discount.
If someone from Larian can confirm the downloadable FOV works with the Steam version it would clear this up. It says on their website "requires Ego Draconis DVD" which would imply it only works with boxed versions.
I was prettyy annoyed at first that I could not continue into FOV from my ED saves but it was worth playing through again.