I am indeed British (contrary to popular belief amongst the most perceptive logicians

), but the point still stands. The UK executive branch can influence (and has influenced) judicial due process where it's perceived as being in the national interest.
A case in point was the early release of the convicted Lockerbie bomber Abdelbaset Al-Megrahi. The UK govt allegedly put great behind-the-scenes pressure upon the Scottish Executive (the legislative body which controlled Al-Megrahi's incarceration) to release him before there was any chance of him dying in prison from cancer. The reasoning was that the UK wanted to open trade links with Libya (specifically in access to oil), and they felt that Gaddafi would sever all trade links if Al-Megrahi were to die in a Scottish prison. The Scottish Executive therefore found a tame doctor (after quite afew attempts!) to give a written opinion that Al-Megrahi was likely to die within three months - which gave the necessary reasoning to release him. Unsurprisingly, he was released in August 2009, and he's still alive (but not well) in Libya. And also unsurprisingly, BP is now heavily involved in oil exploration, drilling and refinement in Libya...
So I understand how governments can "bend the rules" if it serves their interests. In the case of Amanda Knox, though, I can't see any clear reason why the US would want to use any of its carrots or sticks to try to influence the judicial process - it's just too small an issue, with no international connotations, to bring any of the big weapons out. Similarly, I can't see any one politician (Cantwell, for example) trying to hold the US executive to ransom over this case - the risks and rewards just wouldn't add up in this case, in my view.