The story looks legit, and references an original story written by the Daily Mail, but RT can produce an original and insightful report sometimes, especially when there is no Kremlin interest in it. Although in this case I expect RT seized on it because they love anything that shows a split between the US and UK.
As for the story, I think there is an element of truth in it, more because China and Asia is a rising concern to the US then concern then anything else, we are not significant in Asia anymore, and despite Vlad's antics he's still many miles away from threatening European Hegemony, so our importance has declined.
But it is interesting how they attach significance for the UK being part of the EU
The memo, prepared by the US Congressional Research Service, also reportedly touches on a potential UK exit from the EU following an ‘Out’ vote in a referendum: "Both the positive and the negative aspects of a prospective life outside the EU are more difficult to foresee," it notes.
They were keen for us to join the EU back in the 70's, yet as one small country in Europe our voice is very small indeed, much smaller inside as part of a proportional voting system then outside using our influence as a 1st world economy and military power. I wonder if their policy interest in our EU membership is more to keep us under control, then to keep continental Europe under control?