Yes, you cant tax the car until you have the vehicle reg cert and an insurance cert.
If ( as happened to me) you get a ticket for no-tax while waiting for all this, then after you have all your docs, tax the car and make sure to pay the arrears back to then you bought the car, and reply to the ticket with copies of all the docs and the ticket should be waived.
If you insist, Your insurance company should issue you with an insurance disk for 2 weeks, until you have the vehicle reg cert. The insurance disk is more important, as the offence of driving uninsured is much more serious.