Lovejoy usually a 2017 car would have been built in Japan around Oct -Nov 2016 and shipped over here such Car will have the latest Nav software at the time of assembly.
The new portal is meant for all Lexus customers irrespective of the age of car, if you continue to have difficulty registering, I think the quickest way to get this resolved is to contact Lexus UK customer service or if you are on Facebook you could write to them via this link. https://www.facebook.com/LexusUK/.
They would probably suggest a trip to your local dealership where am sure someone would help you with the set up. Hope you're enjoying the car though post some pics later.Regards