Free speech protections in the U.K. are much weaker than those in the U.S. in many key respects.
The kind of comment mentioned might give rise to legal action in the U.K., either civil or criminal (although it wouldn't be an open and shut case even in the U.K.), but this would clearly not be a valid basis for criminal proceedings or a civil lawsuit in the U.S. due to strong U.S. protections for free speech under the First Amendment to the U.S. Constitution.
The U.S. would not extradite someone in the U.S. to the U.K. for an offense of this kind, and U.S. courts would not enforce a civil judgment entered in the U.K. against U.S. assets if the judgment was based upon this kind of conduct. Knowing this fact, U.K. officials and lawyers wouldn't even try to do so.
The U.K. might enforce its laws criminally against people present in the U.K., and would enforce a judgment in a civil lawsuit against a U.S. person entered in a U.K. court (that has proper personal jurisdiction over that U.S. person according to U.K. jurisdiction rules) against that U.S. person's U.K. assets.
As a practical matter, it is not the practice of U.K. officials to commence criminal cases in which it can issue arrest warrants that would provide a basis to arrest a U.S. person if that person arrives in the U.K., for this kind of conduct, even though it could.
