I am not sure how much of this has already been said in previous posts and I apoligise if I am stating the obvious or you already know this..
It is true that Sensei Eustace was a Judo and Tomiki practioner. He was also a police officer and went to Japan to study their teaching/methods with a view to developing a system for British police. Of course there is no way that British police could have just lifted the system from Japan, the cultures are so vastly different and the vast majority of British police have had no previous experience of any martial art. Also British police officers generally would never train like the Japanese police do, maybe one or two sessions a year for some, not like a session a day in Japan.
As you say in Japan 'Taiho' is simply a description for a police system that incorporates elements of existing martial arts. (Aikido, Judo, JuJitsu etc..) Most Japanese police officers will already be proficient in at leats one martial art. My understanding is (I was very young when he originally taught this!) that he took the Japanese program and taught the relevant parts as a sytem. UK practioners therefore trained and graded even if they had no other grade in any other martial art (although of course some did, like Sensei Eustace and Sensei Burton).
The system includes many recognised techiniques and katas from the other arts(Randori no kata for example) but also police specific techiniques (keibo arresting moves, transport wrist locks and the like).
I think basically that in Japan they do not need a grading system in Taiho since they have their grades in their own arts, utilised in the practice of Taiho. Here in the UK we obviously needed some way to grade practioners and Sensei Eustace developed the grading system.