It's nicknamed "the convent" in France because you virtually have no social life during two years. Generally, it's 8am-8pm 5 days a week, and you have "khôles" (exams) on Saturdays, or at least, that's how my school did it. You have three strands, the Literary one (Hypokhâgne + Khâgne), the Economics one (Prépa HEC) and the science one. They've all got substrands, so you can focus on Physics or Maths in the science one, or you can go for a half science-half lit kind of thing (I forgot the name). It's extremely intensive, and you have a wide range of subjects you need to work on, you don't just do maths or just languages, it's a mix. I've met some people on their year abroad at Oxbridge, or students who'd moved to the UK after a prépa, and they all said it was far easier. There isn't as much pressure, you've got more freedom, not as much is asked of you, you don't end up with exams every week, it's a lot more chilled out.
As far as I'm aware, Oxbridge does not allow transfers whatsoever, so even the prestige associated with prépas (assuming you even get the grades to benefit from it) wouldn't ease your way into Oxford of Cambridge, you'd have to apply the usual way, especially as you can only get the Senior/Affiliated student status after doing a whole degree. In France, unis know prépas are definitely worth two years of university, but if you want my opinion, it's not worth going through the hassle of it to then not get into a GE. I'm sure you know that Oxbridge want students who, they believe, will suit the tutorial system, and although prépas are intense, competitive and hard-work, it doesn't mean that everyone is suitable for the Oxbridge teaching, and they may not have shown enough passion for their subject. Being good at it is one thing, having the passion for it is another.
The Baccalauréat system generally teaches us to put up with all kind of crap we don't really give a damn about, I did 13 subjects, of which I was only really interested in one, but I didn't have the choice of not doing that many. We learnt to work by ourselves and not give up, but we also know that it's because it'll get us a job at the end of the path. A vast majority of people do the Scientific strand of the Bac because it's seen as the most prestigious, and an idiotic number of them end up deciding that actually, they'd rather do humanities at uni, but thought the literary strand wouldn't be good enough. I'm not sure French students really care about their subjects, we pick the job and do the studying after, whereas in the UK, you tend to go for what you like and see what you can get after. And this could mean that we don't have as much as a passion for a subject as Bris would.
All in all, you're better off going to Oxford this autumn if it's your aim, trying the prépas system to improve your French is a bit masochistic, if you ask me
I don't really understand why you got negged for your post, but oh well…
I don't think your skin colour would be a problem, you just need to be careful as you would everywhere else in the world! French seem to have a problem with Islam at the moment, so keep an eye on what's going on, because with the recent niqab ban, there may be more stupid laws to come…