And in practice, what would that look like?
Obviously, it would require use of the UK's airforce, which is about 10% of the size of the US's and not as sophisticated. On top of that, about a third of the UK's airforce (including most its more powerful aircraft) can essentially be switched off by the US.
The naval situation may be marginally better, on paper, but would be of less practical use.
Then there's the fact that most aspects of British life (to the extent they involve computing, either at point of use or back-end admin) rely on American software to some extent or another, leaving the UK extremely vulnerable to US cyber warfare. Banks, hospitals, telecommunications, energy, etc, etc, all disabled.
And even if the UK overcame all that an pushed on regardless, the US could completely level the UK, through a nuclear attack, in less than an hour.