I'm sorry a draft is a pro forma invoice if you want to talk about documents. A pro forma invoice is not an invoice. A pro forma is never acceptable for VAT. Only an invoice is. You can argue until you are blue in the face but HMRC, UK Tax Authority, are clear about the difference and therefore we have to be clear about the difference.
You don't address my initial point about how you can create a gap in invoice numbering simply by hitting the sales invoice option by mistake. Using sales orders doesn't address that at all.
I'm delighted for those countries that don't have to deal with VAT but I do.