VAT invoices can only be issued by persons who are REGISTERED for VAT.
However non-registration does NOT relieve the person who was intially refused registration (pending further requested information to be provided), from the requirement to make VAT returns !!
Regulation 25 of the 1995 VAT Regulations, requires EVERY person "..who is registered... or who is required to be registered...., to make VAT returns.
How, in practise, this would be done poses an interesting dilemma.