(1) UK: (a) Mostly not if you want to work there in a law firm, unless the firm in question viewed it as a sign of commitment to the UK from someone who wanted to make a lateral move (unlikely, as most UK firms view LLMs as academic degrees) (though an LLM + PhD combination could help you get into UK academia (or possibly even a barristers' chambers, if you had some strong combination like Oxford BCL + Cambridge PhD)); and (b) a good UK LLM could set you apart from the pack if you decided to come back, or make a move to a third country.

(2) U.S.: (a) Not worth it at all for working there (U.S. lawyers seem to have a similar-to-NLU 'Lallum' mentality); and (b) not worth it academically (unless you're doing something very specific, e.g., Legal Philosophy at UCLA), as the difficulty of U.S. LLMs is lower than that of their UK counterparts - they are less academically heavy, less theoretical etc. The catch with U.S. LLMs is that, unlike the UK, you can't even enter the U.S. academic job market with an LLM + SJD (U.S. SJD is their PhD in law) combination, because:

(i) SJD admissions at most good U.S. law schools are very selective, as they only select from their own LLM students, maximum selecting one or two students per year (and sometimes none);

(ii) the SJD isn't a terminal degree designed for U.S. law students, but a doctoral degree designed for foreign students who would want to enter the academic job market in their own country or a third country (U.S. law schools almost never hire their own SJD graduates, to my knowledge), but with U.S. qualifications under their belt; and

(iii) if you can somehow get past (i) and (ii), you may not end up getting hired at anything like the best U.S. law schools.