I've never heard of any protocol and wouldn't really expect there to be one. At one level then, yes, it might be argued that a HT would do well to consult the class teacher before raising any concerns, as that might give a fuller picture. But, equally, I guess it could be argued that whether and how to raise questions with parents about any aspect of any child's experience at school is very much a matter for the head's professional judgement. The head, after all, is the senior professional in the school.
Thinking about your circumstances: do you know that the head hadn't in fact spoken beforehand to the class teacher (apologies if you mentioned this in earlier posts - I haven't re-read them)? And isn't the issue here that the head hasn't said that your son has special needs and hasn't (apart from mentioning the book-biting and solitary play) indicated that there are any significant problems?
I am trying to be constructive here but I wonder what you might have been posting if it had turned out that the SENCO had been consulted. Might you not then be arguing that the head should have brought you in at an earlier stage and that it was premature to consult SENCO before you?
As Romy (I think) said, it does rather sound as if the head might have been fishing for anything to explain the book-biting and aggressive play. If the head was concerned about aggression in the playground then I think we have to assume that they've seen enough playground behaviour over the years to know what is run-of-the-mill rough and tumble and what isn't. Similarly, I think chewing books is unusual enough for the head at least to be curious about whether there's some underlying issue. All in all, I don't think the head can be castigated for trying to understand your son better - isn't that what we all want from our children's schools? If the head has reached the wrong conclusions, that's a different issue.
Finally, I also think it needs to be said that even if the head had said that your son was having problems at school or possibly had special needs, that isn't an insult. Again, I may have got this wrong, but you do seem to be indignant at any implication that your son might be having a difficult time or have special needs. I can understand your frustration if these things were hinted at but not fully developed, but identifying any child as having special needs is not a criticism of the child or the parent - it's a means towards meeting that child's particular needs and ensuring they get the best out of their time at school.
Sorry, this post is too long. I ramble when I'm tired.