So im sort of guessing here so take this with a grain of salt.
but I imagine with how countries work with each other the uk would first fine the company if the company denies to pay the fine then the next step is trying to work with law enforcement in that country.
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"The GDPR regulatory authorities have the power to enforce their standards, thanks to international treaties and agreements"
but lets assume that the usa or whatever country the company is from decides not to comply to the agreement (which could effect their ability to enforce their laws in the uk) then the next step would most likely be what you said block ip's from accessing the site or even dns services as well.
So for why would these companies comply could be because they want to keep a UK/EU market
and even if they don't comply their country may force them to so that they can keep their own agreements and treaties from being blocked aswell.
That being said it's possible that the USA may view the uk's rules as too strict and could end up effecting usa citizens rights and step in too stop it on that account. but considering the usa is already headed in the same direction with it's laws I doubt that's gonna happen