I may be horribly wrong, but I believe that all children use phonics as the basis of their reading, it's just that some internalise the patterns more quickly so they just read 'naturally'. As adults we also 'sound' unfamiliar words out.
A good reader looking at 'blurp' will still be breaking the word up into its component sounds - they will just doing it quickly! I have to read 'blurp' or 'sweck' using phonics!
If (when?) the results of the phonics check are analysed, it will highlight (for example) a child who says b-l-u-p for blurp, but correctly reads slerp for slerp, and it will inform the teacher that he hasn't yet learnt to read the phonic equivalent of the 'er' sound.
Personally, I think the 'phonics check' is a (political) compromise to the regular, half-termly assessment that takes place in RWI. In RWI, in order to get to the next group a child has to read a mixture of:
*focus sounds (not in words)
*real words containing the focus sounds
*made-up words containing the focus sounds
So, for instance, to get to group B, a child has to read all letter sounds; to get to group C, they have to read sh, ch, can, tin, fup, bup (for instance) and so on, to group J when all 44 sounds will have been learnt and tested.
This phonics check does that in a kind of non-specific, half-hearted way, but it should do its job and highlight the children who need intervention.