they can put out arrest warrants and fines

Of course they can, but they're unenforceable if the instance's operator has no presence nor assets in the UK. That's why I wrote "forced" in bold. They can issue shit, but that's where it stops.

and possibly have the person extradited to the UK

According to Wikipedia, extradition requires that the crime is such in BOTH countries. So no, I don't see that being an option here.