VINs tend to become available quite early as part of the firming up the delivery date, Reg's however are much closer to the actual delivery. I ordered from inventory so it might be slightly different, but I found my reg on the invoice document, they then sent me a different Reg 2 days before collection which was different and the Reg the car was delivered with - it required a update with the insurance company which was painless. The Reg is linked to the delivery location/region, so until your delivery location is pinned down I doubt they can reliably issue a Reg anyway.