I'm not sure about graphic design, but in general I think St. Martins, Kingston and LCC would be hardest out of those - not sure about the FDA courses though. I visited Kingston on an open day a few weeks ago and it's pretty good - good reputation, dedicated tutors, good facilities. The graphic design studios are pretty spacious, and its end of year show was pretty impressive. It also only accepts first choice route b on its BA courses, which is a good sign. Obviously St. Martins and LCC have awesome reputations because they're part of the University of the Arts London, but not sure what they're like. I haven't heard good things about Metropolitan, though I haven't been there myself. Middlesex sounds pretty good, though I don't think it has the reputation of the others.
I'd go visit them all, if I were you - though I think I'd pick St. Martins or Kingston out of those - St. Martins for the rep, Kingston for everything else. Good luck with your decision!