tatanab wrote: ... it is not something they dreamed up in 1983. I recall from a few decades ago that this particulate part of the requirements has been in place since the late 1940s or early 1950s and simply carried over .
Exactly. That's the point I was trying to make and I'm sorry I didn't make it clearer. I don't think that the legislators dreamed up anything, except two different ways - originally wheel size and then saddle height - to exempt toys. I cannot imagine, for instance that Kirkpatrick Macmillan was able to check any Hobby Horse (Construction and Use) Regulations before he got his thinking cap on.
For long enough the pedal cycle and its predecessors were treated as "carriages," governed by the same rules and that persists, of course in some statutes. (A schoolfriend thought he was clever when he referred to a bicycle as a horseless horse.)
I'm saying that at each stage they formalised what was already customary.