There's no UK/US distinction here.

1.    "who" is correct for both subject and direct object clauses. Old grammar books might insist that only "whom" is correct for object clauses but this is not a standard view now.

2.    "whom" is also correct for direct object clauses.  It is more formal than "who"

3.    only "whom" is correct when it follows a preposition e.g. to whom, from whom, "ask not for whom the bell tolls..". 

4.    "Preposition + whom" is a formal construction and can be replaced with "who + [verb phrase] + preposition"

e.g. Do not ask who the bell tolls for.

You could say "He is someone (...) I admire." where (...) is "whom" but said in a such a very very very soft whisper that no one else can hear it.* That way you wouldn't sound like a fuddy duddy grammarian, but at the same time, you would be grammatically correct and pass English exams. How's that for having your cake and eating it, too?

*thank you, Phil. I see you have travelled that path already. Or not.

@Michael Chambers 

is this a hangover from when English had cases? I always noticed that WHOM seems correct when one would put it in the Dativ case, like you said- to whom, from whom, etc. 

its a handy reference point, cases. I tell my students from Germany to think of he as Nominativ, and him as Akkusativ, for example.