There's no problem jumping grades... it all music, there's no rule saying you should learn pieces in a given order of difficulty. Paticularly since if you take Grade six pieces, you'll find that each piece has different difficulties, so you may be well prepared for some Grade 8 difficulties, and poorly prepared for others.
Just realise that the harder the material gets the more glaringly obvious your technical flaws become. - Its easy to learn bad habits if you shoot ahead of yourself without an experienced eye watching over your shoulder. And make sure you don't hurt yourself by trying to force the skills into your hand by over practicing. If something is particularly difficult and you dont seem to be improving after practice you're probably moving in a bad way and need to fix that, not just practice more.