Soil/Waste pipe question

I am a friend of mine has asked me to post this question on the forum.

He is having som major work done on his property, leading to 3 en suites and a 1 bathroom to the rear of his property. He wants to know whish soil/vent pipe to use and is there regs related to the type that is used.

EG can he use the brown underground pipe and the push fit fittings to carry the waste from the 4 toilets to soil stack (which is already in place)
as it works out quite a bit cheaper, but does not want to be told by the building inspector that it has to ripped out and replaced with grey or black.

Thanks

Ug pipe in its nature is for under ground. it is not suitable for sunlight just as is blue mdpe pipe isnt. above ground inside it may be ok but id check with building control 1st as i wont risk it . UG is UG so is it worth the risk.