Both the Foreign Office and the US State Department appeared to accept that she no longer had diplomatic immunity, and she wasn't a registered diplomat anyway -- she was the Intelligence officer's wife. Or at least that's what the Foreign Secretary was claiming at the tail end of last year. The surprise is either of them getting immunity for an airbase intelligence role outside London in the first place. I've seen several reports questioning whether the immunity was valid at all.

But having returned to the USA there is no longer any immunity. A crime is still there to be investigated and prosecuted, an extradition could proceed.

Additionally US-UK extradition agreements both under Thatcher and later under Blair have been notable for their imbalance, but that seems rather academic here.

A balanced discussion of the issues underlying: https://www.diplomacy.edu/blog/sacoolas-affair-diplomatic-im...