American Universities vs. UCAS Universities

Hi, I’m a junior in an American high school right now, and I am having trouble gauging what colleges exactly I would have the chance of getting into… I know that American universities-- especially the top ones-- look at the application process really holistically (grades, tests, extra-curricular activities, leadership, awards, etc.) but that a lot of British universities focus mostly on grades and standardized test scores, rather than EC’s and leadership positions. I feel as though my grades are good enough to get into a good college, but my EC’s and leadership positions lack terribly. For this reason, I am not sure if I should be gearing my mind toward British schools or American schools and what tier schools I would have a chance of getting into? If someone could give me some advice, I would really appreciate it!

My classes-- [Freshman Year] Honors Bio (106-106), Japanese (100-100), Honors Literature (98-103), AP Government (105-105), Spanish 2 Honors (105-106), Geometry Honors (103-103) – [Sophomore Year] Honors Literature (100-104), Honors Chemistry (100-104), Spanish 3 Honors (100-106), AP Physics 1 (98-100), AP World History (102-103), Japanese 2 (100), Japanese 3 (99), Chinese I (100) – [Junior Year] AP Art History, AP European History, AP Calculus BC, AP Lang, AP Physics 2, Spanish IV Honors, Chinese II, AP Japanese

Extra-Curricular-- Beta Club, food center volunteering (~40 hours), middle school tutoring (~20 hours), taekwondo demonstration team, 2 gold medals in taekwondo tournaments, volunteer program in China next summer

Test Scores-- SAT World History (800), plan to take SAT math 2, physics, literature, Spanish, and Japanese, plan to take the ACT (getting ~32 on practice tests, but aiming higher)

So, as you can see, I don’t really have leadership positions or much awards or anything, but my grades aren’t that bad (especially if I can keep them up). Therefore, would I have a better chance of applying to good schools in the UK (e.g. UCL, KCL) or getting into good schools here (e.g. Dartmouth, UCLA)? Or am I completely off on those questions (i.e. no way I would be able to get into any of schools that prestigious?) I’m apologize for the length of this post, and I’m not trying show of my grades or anything-- I really have no idea where I stand. Thanks for any responses!!

P.S. I’m hoping to study international relations and/or linguistics in college.
P.P.S. For anyone who might know-- Do British universities allow double-majors and study abroad?