Can you cite said law for us?

It's the Online Safety Act. As the government says about the OSA:

"Ofcom is the independent regulator for Online Safety. [...] Ofcom has strong enforcement powers"

https://www.gov.uk/government/collections/online-safety-act

Okay, so what does Ofcom say?

"It doesn’t matter where you or your business is based. The new rules will apply to you (or your business) if the service you provide has a significant number of users in the UK, or if the UK is a target market."

https://www.ofcom.org.uk/online-safety/illegal-and-harmful-c...

I’m sure that they can write that. But their actual enforcement mechanism is nonexistent. No country is going to work with the UK to arrest someone that does that when the same thing isn’t illegal under their own law.