Registration etc is normally done while the vehicles are in transit, on the boat.
No registrations are "done" while vehicles are in Transit or even when they arrive at Southampton or Tesla delivery centre or in case of personal deliveries, at buyer's home address. Registration with DVLA only occurs the day the vehicle is delivered and after the buyer signs their respective acceptance document confirming delivery and accepting of vehicle. Prior to delivery, Tesla like all automotive manufacturers receive a large preallocation of registration numbers/marks (VRN) and Tesla in turn assign these VRNs to matched vehicles but these VRNs are only registered (activated) after above occurs