Re: "On April 21, the High Court refused Dr Lynch's permission to appeal his extradition"
I don't believe you're correct as to the regulation point: a US company relied on representations by a UK company which were fraudulent. Yes, the issue of fraudulent company accounts is a purely UK issue with respect to UK company laws, but this isn't an issue of "did Autonomy break UK regulations by filing faked accounts?" but rather "did Autonomy con HP into paying too much money for the company by generating a fake set of books?"
HP, of course, is well aware of UK accounting regulations and policies, having UK subsidiaries (a quick search indicates about 4 HPE ones, and 26 HP ones). So it is entirely reasonable to conclude that, although faking the audit would be a criminal matter _in the UK only_, using the faked audit to fraudulently close a deal with a US one is very much a US issue and one in which HP might legitimately conclude that the audited UK accounts were reliable.
I certainly agree that there are issues as to why Lynch didn't get prosecuted in the UK, but there is more than one crime at issue here. If the US was trying Lynch for cooking the books, I would agree that that would inappropriate... but he isn't. He is being prosecuted for a crime that, effectively, took place in Palo Alto, CA: the representations made by Lynch to HP in order to personally pocket a lot of cash.
However, on a technical point, hearsay is NOT generally admissible in a US court. Or rather, "out-of-court statement offered to prove the truth of whatever it asserts, which is then offered in evidence to prove the truth of the matter" (i.e. hearsay) isn't, although there are exceptions (death-bed declarations and so on). Of course, there are (minor) differences in the Federal Rules of Evidence with regards to criminal vs civil trials, and the issue of immunity from prosecution, and that's why in the USA civil trials will typically be stayed pending the resolution of the criminal case.
There's a press release here https://www.justice.gov/usao-ndca/pr/michael-richard-lynch-former-ceo-autonomy-corporation-makes-appearance-federal-court that describes the outline of the case against him. You'll see it just asserts that Lynch cooked the books to show a higher value and prospects than were supportable, but does not go into detail as to how the cooking occurred!