Ironically, the UK did look at the research that was done, but only at the phonics element.
There was a good deal of work done in the US on phonics, in the context of the debate between whole word teaching of reading and phonics based teaching. The research was carried out on children in the US who would be starting formal education, having spent the half days of their Kindergarten year playing, doing art, using scissors, pasting crafts together, singing songs, having stories read to them, doing nature walks, doing show and tell, learning to share classroom materials, to clean up after themselves, to put on their own outerwear, hats, mittens, snow bibs, boots, etc.
So basically, the research was done with children aged 6, or 5 at the youngest..
The fact that the children were older than typical British children in Reception was lost on the British experts, who may not have been familiar with school starting age in the US and assumed it was 4 as in the UK. All they noted was that phonics emerged as the best way to teach reading, and set about justifying the expense of providing classrooms and teachers for 4 year olds by making them learn a skill that could be measured.