As suggested, I contacted Koch & Bergfeld. They kindly responded as follows:
"the number 17500 tells us that the design of your teapot is from the year 1890.
The teapot has been manufactured up to the 1930´s but unfortunately it is impossible to tell when this particular one has been made.
Concerning the design, we assume an English direction, maybe Christopher Dresser who is known for a very similar design…"
This information from the manufacturer concurs with the expertise of this forum! With the non-ivory insulators, it would seem to me that its actual date of manufacture probably sits closer to 1930 than 1890, as has already been said, but given K&B's comments, not as late as the 1950s.
As for the type of insulator, the obvious candidate would be coloured bakelite (which was patented in 1909), since most of the other plastics that I'm aware of that were developed by then would be too soft (PVC, celluloid, polyethelene). Even polystyrene in its solid form, which was available in the early 1930's, melts at just above 100 °C and so may have been considered too close for comfort for a vessel holding boiling water. Also I would expect some degradation of polystyrene over time, but the insulators have not degraded.
Anyway, unless someone is able to add anything else, I feel I have the answers I was looking for.
Many thanks to all the contributors,