The 'children who are failing' are not ones who could read in reception and then weren't given enough spelling tests, believe me.
The guidelines and good practise are based on reseach and studies into how children learn at a young age. Yes, some are bound to be ready for something more formal and there shouldn't be a ban, but it should definitely be the exception and not the rule.
Even with children who seem 'ready' there is no need to rush, there is no advantage, research demonstrates, you do not necessarily 'get further' by starting earlier, in fact often the opposite.
This is why most other countries start formal teaching much leter than us. They base it on the research.
The foundation is trying to rebalnec our too formal early education slighly, but is battling against some teachers, and many parents, who are stil too eager to do us much as possible as early as possible.
belive me, it damages many and has advantages for very few, it is really not necessary.
And therefore BAD PRACTISE.