you might as well apply to both (as in apply both this year and apply to usa/canada)
The USA and canada have later deadlines for applications, usually around january-march, so you'll lose nothing by applying here in the uk also.
However be warned, don't think not getting good enough grades for the UK course you want is magically going to be looked over if you apply to America. They're not eagerly looking to welcome you just because you're from england. They have just as much of a high standard, depening on the university/college you apply to in those countries.
So If you think you wont get into a course in the UK, its very unlikely you'll randomly get into a similar course in america. home students always come first.
Dont forget that they're going to be american students with higher grades applying for the same courses and though most unis have a set number of places for foreign students, American/canadian students are going to be considered more seriously than a foreign student, especially since they're grading system is completely different. Not to mention the cost and debt you will be in. Its unlikely you'll get a scholarship (but not impossible) and unless you do, you're going to have to consider the ****load of money required for you to study at an american university. for home (american) students, this can range from $14,000 to $30,000 a year. and it'll be twice that for international students.
Do you have that kind of money? Not to mention the living costs, costs of travel, having to get a student visa...not having family or financial support. no NHS.
In light of that, i'm not trying to discourage from applying to study in america, just be warned of the things you will have to seriously consider and be aware of.
i know some people like to think of oxbridge as the only relevant uni, with all other unis being inferior, but that simply isn't the case. oxford and cambridge are top, but so are many others.
A universities status isn't determined by the university itself but by its students.
The invention of MRI as we know it came from the work of a professor at nottingham university.
think carefully and don't let elitist myths about no other unis being as good as oxbridge wear you down. also dont forget, even if you dont get to oxford as an undergraduate, there are a lot of graduate courses available in cambridge and oxford if you want to pursue a masters or phd later in life.