Do you home educate, Ffsffsffsffsffs? I don't get the impression you have much experience of home education.
"Unless you are teaching him all the dull grammar, vocabulary and exploring different writing styles, yes, simply just writing a story every day WILL damage your sons progress (and could prevent him being able to pass entry tests for private schooling in y7)"
I don't know what the entry tests are like, so I'm sure it would be a good idea for the OP to get familiar with them and prepare for them. But it doesn't have to be a hard slog. When you replace the school day (including travel to school, waiting for the teacher's attention, work which is too easy/difficult/uninteresting) with time focused on the child's individual needs and plenty of one-to-one attention, efficiency soars. It is common for home educated kids to follow their interests for years, then concentrate on acquiring the skills they need for a particular goal - passing an exam, say - and whiz through them in a few months. It doesn't have to be a long dull process.
If the OP's son is reading widely, he will encounter and absorb good grammar and vocabulary. Trying out different writing styles can be interesting, and may be sensible in the run-up to an exam which requires that skill. My eldest, who is academically average, did no formal study of English (or any other subject) until shortly before their English Language IGCSE, They found the exam straightforward and got an excellent result. For example, having read thousands of newspaper articles, they were very familiar with the style of newspaper articles. In preparation for the exam, they quickly swotted up on a few characteristics of newspaper articles which they hadn't previously noticed. After that, they could churn out newspaper articles. School turns some skills into chores by forcing them upon kids who are too young and don't see a reason to learn them yet.
In the absence of a long school day followed by homework, home educated kids have the time to do much more reading for pleasure. My kid tried school for a term in Y5 and then left. This was for several reasons, but mainly because school was so inefficient that it left them without the time to read, play, see friends or do hobbies. I found it ironic that while at school, my child struggled to log even 20 minutes of daily "free reading" in their school reading diary. At the end of a school day they were too busy, too tired, and too fed up with scheduled activities to squeeze it in. Before and after their stint at school, they had read for several hours a day!
"I take it he's classed as vulnerable, which does set my spider senses tingling as to what provision you are hoping to give at home (plus if there are any safeguarding concerns or other agencies involved with your child)"
Your spider senses are overexcitable. Vast swathes of children are being classed as vulnerable, with the implication that they are better off at school. It's great that they are being OFFERED school places, because some of them do need school. But many vulnerable kids suffer through being at school, especially now that school staff are so overstretched by the requirement to educate kids at home as well as those at school. Parents are in a good position to decide this for themselves. It's offensive to imply that the OP wouldn't have thought this through.
"It really is quite simple to deregister in normal circumstances, does the school have concerns?"
It should be simple. It has always been the case that many heads don't know the law, are alarmed at the idea of children being educated in a different way, and fear that deregistration reflects badly on their school's provision - which in some cases it does. Some LAs are currently telling schools to adopt the illegal practice of delaying deregistration while trying to dissuade parents from home educating.
"I hate to say it, but the academic achievement of home-Ed kids is outstanding in only a very small number of cases."
I hate to say it, but there is no data whatsoever to support your view. What's more, even if true, it would be irrelevant to the question of whether a particular child would receive a better education at home than at school. For several reasons, it would be hard to collect meaningful data on the effect of home education on children. One reason is that as a self-selected (or parent-selected) group, they are unlikely to be representative of the general population. For example, surveys of parents who have deregistered children in recent years indicate that the most common reason for removing children from school is poor provision for their special needs. This suggests that kids with SEN make up a significant proportion of the home ed population, which would skew the statistics.
My youngest child, like many of her home educated friends, has significant special needs. She will not have done any GCSEs by 16. Would this demonstrate that those kids missed out by not attending school?