I'm more worried about the US deciding to invade the UK than the UK deciding to invade the US, but a plausible scenario can me made out about twitter and other US owned media. These media deliberately enable breach of UK court injunctions. They do so by speech directed specifically at the UK, so the UK courts have jurisdiction. That is a contempt of court and the UK courts do have the power to order the arrest of the contemnors.

Shall we send a gunboat?