If it was an ad hoc movement, once it achieved its aims the people who supported it will revert back to a party with more relevant national goals and ambitions. I don’t know what UKIP even stands for as an acronym, but people don’t spend a life voting on foreign entanglements. UKIP made its point with the EU vote. It didn’t transition to any meaningful domestic policy that the voters could support long term.
This generally happens in any 2-party country (US, UK, France et al), if a 3rd party rises to enough strength in the polls to determine the outcome then one of the 2 major parties simply usurps (or steals) its thunder by incorporating those idea that make the 3rd party popular as their own. Doesn’t mean they will actually follow through with those ideas. Some countries with Parliamentary systems do have more than 3 parties in their legislature but often the 3rd party gains just enough power to matter at one critical point in time.
UKIP didn’t stand just for BREXIT. They were also the only party actively speaking out against unlimited immigration (especially muzzies of course).
Guess the voters don’t mind being overrun by third world savages all that much after all.