The big range in reading ability in England comes even though children get started on phonics in England at 4 and it has always been there, all through the history of measuring children's reading performance, with or without systematic phonics instruction, so there is really no point in doing something that is probably not going to result in any general improvement in the long term, but which can take time in the classroom better spent focusing other things.
There are a lot of factors besides teaching or not teaching phonics at age 4 that go into creating good or poor readers at 7 or 10 or beyond. Even children in the same family can have a wide range of reading ability or motivation. My DD1 and DD2 read from an early age, before school, having basically picked it up themselves. DS learned himself while in kindergarten in the US (age 5ish), and the other 2 DDs learned somewhere between them. Trying to get DS to read books (apart from Captain Underpants) was like pulling teeth, but he read every technical description of his favourite subjects that he could get his hands on (planes and flight), then moved on to weapons, tanks, artillery, military history, etc. DDs 1 and 2 read their way through their entire lives to this point and always have a book going. DDs 3 and 4 never opened a book after getting through the basic decoding skills stage, but suddenly at age 9 or 10 started in on the deep end, having skipped the basic chapter books and Look I Can Read sort of titles. I kept on reading to them (entire Narnia series and others) through the drought.
I would like to postpone the perception of obvious differences between children in the area of reading until the children themselves are better able to deal with the fact that that difference exists, until they have a firmer grasp of who they are as people and will not be put off trying to improve by an internal self definition formed by their early experience of failure in this area..
I have encountered parents who get all tied up in knots about reading, mainly in places where early reading is taught and therefore early learning is expected and therefore seen as the norm -- I think there's a vicious circle there that does the affected children no good at all. OTOH, I have seen some parents in the US get very worried if a child does not appear to have good hand eye co-ordination or seems not to be cut out for contact sports, which is where it's at for many parents there.