You are correct, but it is tanking. No politician is going to stand up and take credit for a tanking economy. They will blame whatever they can for their failures. UK's failures will be blamed on US credit downgrade.
Perhaps some leaders from other countries will stoop so low as to blame a tsunami, or an earthquake, or perhaps even a Tea Party for their economic malaise. Yeah, I know. That last statement is too far-fectched for anyone to believe. Those words will never be spoken.
The fundamental fact is that all the worlds economies are now very tightly tied to each other, and if one goes it affects all the others (some not as much, obviously). UK politicians make errors, and sometimes they are not able to make the correct moves for various domestic political reasons, but even if they get everything right the economy might still tank, because so much of what happens in the global economy is beyond their direct control. US politicians have got more room for manoeuvre, because the US economy is a bigger player.