Long shot - do I have a chance? Canadian considering applying to the UK

Hello everyone - I am a Canadian citizen who is considering applying to universities in the UK, preferably one of the top 5. My high school academic record is rather subpar (four above 90%, three above 85%, two above 80%, four above 70% and one below 70% in Grade 12), and certainly not one within range of the desired universities (competitive applicants to Oxford, for example, typically have an overall average in the low-to-mid 90’s, compared to mine in the low-to-mid 80’s). That being said, my top marks are concentrated in history, languages and literature, with the lowest marks being in mathematics; I will most likely apply for a history degree, considering my interests. I also have three 5’s and three 3’s from Advanced Placement exams, with the best scores in history as well. I should note that I am not currently in high school or university and hence have quite a bit of free time available if required, so if taking A-Levels (or other exams, like IB) will help my prospects, I am strongly willing and able to study for them. My questions are thus:

(1) Are my prospects for a top UK university realistic now? How about if I score well on A-Levels (or other exams)?
(2) Will taking A-Levels (or other exams) make a difference? If so, which exams do you recommend?

Thank you very much for your help in advance! Any other advice would be greatly appreciated.

Absolute minimum entry requirements (competitive applicants typically much higher):
Oxford: 5 Grade 12 courses at 85% or above

Cambridge: English 12 + 5 top courses at 90% or above, plus five AP 5’s
LSE: 6 Grade 12 courses at 86% or above
KCL: 5 Grade 12 courses at 85% or above for AAA equivalent, 90% or above for A*AA equivalent
UCL: 5 Grade 12 courses at 83% or above

What are the AP exams with 5s? Are you only considering London unis?

Why not Durham or Exeter? Very highly ranked in History.

jupiter98: Thanks for your reply - the AP exams with 5s are US History, European History and French Language and Literature. I am not only considering universities in the London region; however, I am aiming for the best universities I can possibly have a chance at, hence my uncertainty at my chances at upper-tier universities.

VickiSoCal: I am considering St Andrews, Durham, and maybe a couple others as well. I’m not sure if course-specific rankings matter too much to employers.

What kind if job will you be looking for and where?

I think it would make sense for you to take SAT/ACT and SAT2 literature and apply with american qualifications. You should have a good chance at KCL, Durham, Exeter, etc.

jupiter98: I’ve already taken those tests, did OK but I don’t think retakes are looked upon too well.

VickiSoCal: Preferably finance, anywhere.