Appreciate it's only an educated guess, but I've not seen one this like before ( the large yellow ). Maybe an old coal gas pipe? The house is from ~1900 and had a retrofitted gas system which was ~1" steel screwed pipes inside the house where they popped out of the poured concrete. (It's now a standard copper system with external meter). So I assumed underneath the floor it would also have been black painted threaded steel (indeed you can see such a pipe running alongside).
The pipe in question I also found running through the kitchen and it was buried naked in concrete (in these photos further away it has a yellow plastic coating). It's about 10cm dia and I thiiiink very thick copper (it's completely devoid of corrosion and sounds high pitched solid when tapped, not like lead piping dull thud). It's completely immobile.
Unfortunately ALL our services were helpfully buried under a poured concrete stair pad at some point so we can't identify which goes where.
Location is South West London. This is just a curiosity satisfaction, I'm not intending to alter it! Thank you :D
