I’ve had these things buried at various points in the garden to support beds etc.
The previous owner used them for all sorts. Particular pain the backside to dig out of clay soil!
I had an asbestos company look at them who advised they were fibreglass.
They have various bolt holes on them and look like they were from a prefabricated structure of sorts. The previous owner of the house had worked in aerospace at one time or other as this estate had been built for those workers specifically, perhaps he got them from there? Be interested to know.
I’m sure someone on here has an idea
Best regards
Adam