well, I know that a good 2:1 from Cambridge will generally be enough to get you into their MPhil programme applicants from other universities.
I don't know about the film elements. Its not unusual for people to shift disciplines at postgrad level. If you had a good research proposal, then I can't see that the film element would necessarily damage you. After all, it would certainly be reasonable to talk about film within a straight English undergrad essay, so you could almost certainly make a case for its being a positive thing. I guess, though, that it might be more important to get the first.
But, I wouldn't pick your undergraduate degree on the basis of speculation about postgrad. Pick the place/course which most appeals - after all, you have a better chance of doing well if you're doing what you want to be doing. You can't know now what you'll want in three years' time. (And, if your heart is set on film, and you decided you wanted to take that further, then maybe Oxbridge wouldn't be the best place for you - not because you're not going to be good enough for them, but because, with rather limited fellows in the 'cultural studies' end of things, they might not have the right people for you to study with.)