For many years, owners wishing to register a Solex have needed to go through the N.O.V.A. process. This involves getting notification from H.M.R.C. that there is no V.A.T. payable on the machine. A phone number has been available to help with those 'difficult to answer' questions but they have now also introduced a webchat facility for those who prefer that method of communication.
See the information under the 'How old is my Solex?' header in the 'Useful H.M.R.C. contact information.' https://www.velosolexclubuk.com/a-useful-h-m-r-c-phone-number