Just like Trump really isn’t a Republican.
Trump could easily win the nomination because all the other Republicans were weak, corrupt and statist. The UK parliamentary system presents a more formidable challenge to a Farage, especially since he left the Tories. He’d have to get back in their good graces with hat in hand, and even if they took him back, he’d have all the other Tory pygmies to compete with for power (assuming he even had the belly for it).