The Sim Provide has worked smoothly for some for years now, just that as time goes on the reliability of the process improves. When moving to the same provider for BB and telephone e.g. BT Retail they should generate and use the LORN internally without the consumer worrying about it.
80/20 is in a sort of final beta phase, so the restrictions also help to test the systems, while gaining customers who might have signed up elsewhere.