One key element of UK<->US relations is that the US can do literally anything and the UK will quietly accept it. Good example is the Anne Sacoolas case, where an NSA operative killed someone in a traffic accident, driving on the wrong side of the road, then fled the country, claiming diplomatic immunity. She avoided any charges, and the US won't extradite her - despite her lying about the immunity and fleeing from the country - because US.
That's absolutely true on a political level. I'm not so sure that it would apply to breaches of judicial assurances. British judges are cranky, have a strong sense of the dignity of their office and role, and would personally face no consequences for poking the US authorities in the eye with a stick.
I'm not saying they couldn't get consent for the UK for a superseding indictment, but it's certainly not as straightforward as dropping one they've prepared earlier and laughing manically.