so you would rather make your decision from someone on the internet than some professional on site considering the project what are you avoiding ??????It has absolutely nothing to do with planning permission. In fact that's almost funny.
Building control don't define pipe diameters; they leave that to the experts. (I've checked).
I was hoping to find a helpful expert on here to... help...
Never mind. One can only ask. I knew it was a long shot.
you have to contact a professional at some point but why not now ?