alt Hacker NewsThe US has more leverage over the UK then perhaps any other country. Largely because of the shared political culture.
They could throw hand grenades into British politics by declassifying embarrassing events involving British soldiers in Iraq, investigating tax issues with labour party donors (many of whom conduct business in the US) or recognising Northern Ireland as part of the Republic of Ireland.
Attacking the British economy would be a political mistake, because a well advised politician would use it as a scapegoat for any economic problem in the UK. Similar with defence - e.g the withdrawal of intelligence cooperation could allow a terrorist attack to be blamed on Trump rather than MI5/MI6 funding stress.
Seems plausible, though I still think the US threatening to do that, let alone actually doing that, is more likely to cause a separation between the US and UK than to be taken seriously (in the sense of getting the UK to change course).
The UK did just go through having Boris Johnson as Prime Minister, after all, who spent his time in office demonstrating that being completely shameless is a viable solution to almost all blackmail — he only fell when there were photos of him partying in the same period he was on TV telling people they couldn't do that or visit dying relatives because COVID lockdown, everything else wrong was basically ignored.