I myself am currently looking at joining the RAF so i might be able to offer some advice.
While I dont think that maths and physics are essential to join the RAF. Obviously they will help a lot so that might be an issue for them as you have said that you seem to excell more in arts/ himanities.
I can tell you that they prefer candidates straight out of A level (18 years old) and the upper limit is 23 years so you can easily fit in a degree if you wanted. I might be confusing myself with the army but im sure the RAF have online recruitment chats- you should check it out.
With regard to educational requirements you can check the website though im not sure if Alevels/ UCAS points will mean anything to a canadian.
I shouldnt think being canadian will be detrimental to your application- it doessay commonwealth countries and even Republic of Ireland oddly enough. Have you thought about the Canadian forces? I've recently been watching a TV show on discovery about the Canadian air force- F18 hornets! Does Canada have an army air corp or fleet air arm?
ollie