Actually, extraterritorial jurisdiction is something that virtually all countries, not least the UK, routinely exercises. Territorial jurisdiction is one of a number of proper bases for jurisdiction recognised under both international and common law (particularly when you look beyond criminal law).
Pardon me, but I am not diverting down the separate rabbit hole of civil law now.
Extraterritorial jurisdiction has for criminal law traditionally been about extending
jurisdiction into the high seas or where no reasonable jurisdiction exists (e.g. Somalia).
It's perfectly in keeping with a country's sovereignty to accede to a
request for extradition - the key point is that it's not mandatory, and to the
extent that it is mandatory, that's only in a contractual sense.
Extradition has traditionally been about returning fugitives
to the country where they committed the crime.
For the UK government to contract away citizens's rights, dating back
to Magna Carta, to a foreign government is a very slippery slope indeed.
And if it feels that it it is obliged to, then sovereignty has been lost.
I know the lawyers won't concede that, but they live in a world of
legal rules that are not necessarily consistent with geopolitical reality.
They also have the habit of extending such rules to maximise
their remit and earnings in a number of quite dangerous ways.
Hence my use of the word traditional.
For instance there are practical issues. A UK citizen is expected to
comply with UK laws. How can a UK citizen be expected to comply with
all jurisdictions' laws? What happens if Iran or Saudi Arabia wants to
extradite you or me for the crime of drinking beer in Australia or the UK.
My point is that it is not reasonable for a person at home in London to
be expected to know of and comply with USA laws federal and/or 51 states.
And I admit that the UK is a culprit too, trying to extend its laws on libel.
I like your down under use of the term "kangaroo court".
In such cases the odds are completely against the defendant, to a US jury he is
a foreigner and the US government will hire a team of the best lawyers they
can find, while the defendant only gets a bargain basement public defender.