The regulation would apply to the UK. UK brands and goods sold in the UK, by established/registered companies the UK. Not possible to sell any retail goods of the sorts w/o a registration in the UK, so stopping them advertising won't be hard.
I don't think US administration would be able to do anything, much like GDPR.
Tarrifs and other threats
What about Mr Beast video sponsored by McDonalds (for the sake of specificity, a new McMuffin available in every country on earth)?
> I don't think US administration would be able to do anything, much like GDPR.
We live in a different world to the mid-late 2010s. For better or worse, I'm fairly confident Musk and Zuckerberg will have input on US trade policy on these issues.