Thank you for posting on the answer forums, With those kinds of speeds you should be getting a very good picture. There are a few things that you should look into and I will post links that might help get that Streaming correctly for you. They will be found below.
If you look at that last article you will see that the Pro HD actually needs a lower bit rate to get a HD picture. If the speed that you are connected on in the UK is low the Pro HD may get a better picture because of this difference.
I hope this helps,
Sling Moderation Team