"Doesn't this really mean that the US agencies have shouldered aside the SFO because they wish to act as judge & jury on any event anywhere in the world which may impact on US interests?"
No, it means that the SFO have had a good look and concluded that there's no material case. My guess is that they probably did find a range of wide-boy sales accounting that all IT firms seem to favour (compared to most other businesses I've worked in), but evidently not the specific allegations of fraud that HP allege. Given HP's dismal incompetence at everything, I'd be very surprised if there accounting was any better than Autonomy's. And let's face it, which of these two companies admitted to running a range of complex shell companies to hide illegal slush fund payments, and got fined $108m last year? That's right, HP.
Having said that, US regulators are renowned as being the most biased referees in the world, so HP will still be hoping that they'll decide that a bit of creative accounting by Autonomy qualifies as large scale fraud, and that will then vindicate HP's useless, useless management.