But are the other 80% best served by it? Can you point me to research which shows the relative reading abilities of say the top 20% (or 30% or 50% or whatever) taught by different methods and which categorically proves that those taught by phonics alone read as at leat as well, or preferably better, than those taught using any other method?
You'll have to define what you mean by 'reading ability', how you propose to show that 80% of the population read to a good standard (given that no-one tests the reading of the whole population and there is no definition of a good standard of reading) and how you account for purportedly 'mixed methods taught' people who have actually been taught phonics at home.
The 80% figure is only based on the percentage of children achieving L4 or above in English at the end of KS2 and we know that this is not a particularly reliable measure. For example, would you expect a child with a L4 to be able to confidently work out what unfamiliar words 'say'? If you did, you would be very short of the mark at the school I work in as a significant number of L4 children can't do this, neither do they have 'reading ages' commensurate with their chronological age. And they can't spell very well.
I would suspect that this is common to most secondary schools.
We could spend hours bandying figures around but the long and short of it is that phonics is the most efficient method we know at present for teaching the greatest number of children to read.
Teaching phonics does no harm to children who would have learned whatever the method while Whole Word and mixed methods does harm at least 1 in 5 children. It is estimated that some 3 -5% of children still struggle to learn to read with phonics, but this is a far fewer than the 20% who fail with whole Word and mixed methods. And, until the learning process is well under way there is no way of telling who these children will be, by which time the harm is done. (As I teach struggling KS3 readers I am pretty well aware of the damage it does to children)
If parents don't like this they have to option to HS.