He is good at appearing prime ministerial, and it is not just appearance.The prime minister is good at looking like a prime minister, and it is not just appearance (presumably meaning he acts like a prime minister too). WTF is that supposed to mean?
If Cameron is good at looking like a prime minister AND it's not just an appearance, isn't he really just saying that he looks like a prime minister because he acts like one (ie the former is the expression of the latter). An equivalent would be to say that my lamp is good at looking like it is emitting light, and it is not just an appearance (because it is switched on and is emitting light). You can't really claim benefit for both can you? Or are there some prime ministers who act in a prime ministerial way but somehow don't appear to do so?
More generally it raises the question of what looking/acting like a prime minister is. Since the sentence implies that it is a set of behaviours which some prime ministers might not achieve, this means that they are not a result of holding the office of prime minister, and, in turn, raises the possibility that non-prime ministers can appear prime ministerial. But then what if we had a run of prime ministers who were not proficient at appearing prime ministerial but, say, chancellors who were. Would we have to say that appearing prime ministerial was to behave like a chancellor?
I can only assume that the rules of 'appearing prime ministerial' must be written down somewhere. Otherwise a cynical type might conclude that this is just another reflection of the good breeding/born to rule type silliness that seems to particularly afflict right-wing columnists when writing about a posh prime minister.