“if the customer is VAT registered and provides you with the number, you have to include this number on YOUR invoice and then you do not have to charge VAT”
I don’t understand this sentence at all. You must have heard him wrong. If two companies are VAT registered (i.e: you and the company you are dealing with) then you obviously do charge that company VAT when you invoice it. It pays the VAT to you and you pay it on to the revenue.