I don't think you get predicted grades at A-level. I got some predicted grades at GCSE, due to me being in hospital, so pretty valid reason, lol. But I was concerned it may happen again during A-levels, many uni's stated that if I didn't sit the exam, they could let me defer the offer but I will still have to sit the exam in my gap year. I think possibly if you missed one exam you might get away with it, but a whole AS or A2, I think you'd just get no result for it. The universities would know, as they get sent our results through UCAS, so they'd get told you didn't show up or whatever and hence got no grade.
And which certificate are you refering to, the one we get on results day declaring results, or the official one you get to prove you have an A-level? It would obviously be on the first one, but you wouldn't even be able to claim the second certificate if you haven't sat all the modules, as you technically wouldn't even have the A-level yet.