Using underground fittings internally

Is it ok to be using brown underground soil pipe internally. I`ve read that grey is required outdoors because it is UV stablised & brown isn`t. I can`t see that a brown soil pipe boxed in internally will get a lot of UV exposure so presumably is ok? Don`t want my local BCO complaining when they see it if I`m wrong !!
John
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Ask your BCO. Most of them are vaguely human.
Also, why? AFAIK grey is about the same cost as brown.
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