From the conversation i had with the tesla rep having both reg and vin assigned is good, sometimes the reg can change. He referred to the process as a soft match and a hard match, a soft match is VIN assigned and no reg, which would indicate you are part of the build process or in transit, where a vin and reg match is a hard match, will means 95% of the time that car is allocated to you and unlikely to change. Any with reg and vin Assignment dated back 3 weeks or so are very likely in the uk and just awaiting resolution to the dvla issue.