RE: What undergrad would be "better"... I think this depends a LOT on where you would be going if you stayed in the UK. for Oxbridge I would say definitely stay, but for a lot of other universities I really don't think you would be "disadvantaged" for going to a Canadian university.
University is a great time to get out and explore the world, and this is a HUGE plus for employers. They like to know that you are willing to take chances, and living abroad for uni is VERY attractive to them. So even if you are weary of going to a school that "isn't as good" would would definitely be gaining an upper hand in that regard. Very useful for interviews and person statements.
It's hard to compare marks-based admission requirements because the standard of grading is different
Also, would you be pursuing a Masters degree afterwards? If so, I think that this is even LESS of an issue, given that you will be getting high enough grades to do so.
I am from Canada, so let me know if you have any other questions.