I haven't really done criminal law yet (A-level, but .. that doesn't really count does it?) but it seems a bit crazy if you plead guilty to a charge, and then suddenly decide you're not guilty after all and appeal in the hope of quashing the conviction.
It's like stealing a fiver from your dad, admitting to it knowing the punishment, and then saying all of a sudden you didn't do it. Would you expect your dad to believe you, unless you had good evidence to show that your admission wasn't true? Fair enough if you get a sentence far hevier than you bargained for (can you appeal purely against a sentence? I don't actually know) but otherwise it doesn't seem to make much sense to me.