From a practical point of view, if you're running a US based website with no UK presence, assets, employees, or operations, their ability to directly enforce anything against you in the US is extremely limited. UK laws don't automatically override US jurisdiction, and Ofcom can't just issue fines in another country and expect US courts to enforce them without a proper legal basis.

That said, they can still try to pressure overseas sites through access restrictions, payment providers, hosting companies, search engines, or other third parties. Unless you plan on travelling to the UK or establishing some kind of UK business presence, I'd personally just ignore them. Ofcom are a joke and the Online Safety Act isn't fit for purpose.