The advice seems pretty clear, if you aint registered and don't charge VAT it dosn't apply to you. If you charge for VAT anyway and have been trousering the difference that's a naughty that will now show up on the clients books and HMRC will come back.
I am not registered and don't charge it, I absorb it in my quote & margin. I quite often state this in any written quote and no, they cannot have my supplier discount anyway and buy it direct. If it's a big enough value they'll get a decent discount buying direct anyway.