You do not need to know the registration or the VIN to put insurance provisional quote in place for a car well ahead of pickup. The only thing that reg no and/or VIN does is help an insurer identify the specification of a vehicle. Its just a quick way, it can be entered manually and the latter is sometimes needed anyway as there may need to be options, wheels, tow, FSD etc declared to the insurer which a VIN or reg no lookup would not even be aware of.
Once reg number and collection date/time is known, then the provisional quote is then firmed up with the insurer and any premium adjustment paid - this premium will have been calculated at time of provisional quote, less any small pro-rate time adjustment. This is often best to do this over the phone or via online chat as by that point, the car will probably not be registered so insurers online car checks will not pick up the car anyway. A human the other end of the chat/phone will know exactly what is needed and allow you to clarify any uncertainties such as exactly when insurance starts/transfers etc.