Hi All
After correspondence with various VW UK dealerships, VW UK customer service, VW Germany customer service... they half committed to a response that the infotainment system (MIB2 unit) is regionally specific to Japan and therefore VW UK may struggle to connect or diagnose faults with it. Multiple dealerships would not even allow me to make a diagnostic booking with them just to see what they could/couldn't scan successfully.
VW 'type approval' eventually said they 'should' be able to connect to our vehicle with ODIS, but because it wasn't originally built for the UK market, their VMS (Vehicle Management System) may not find the registration number or VIN and they confirmed the concern about the infotainment system.
VW confirmed they have no internal process to swap the car to the UK region, no comparison documents to highlight key differences, and no retrofit solutions to align it to a Passat built for the UK market. Literally zero interest from anyone at VW.
It appears that everything I've received from VW though is backwards/nonsense. As I have found independent specialists (such as ADS in Preston) who can connect to all components in the car without any problem at all. They have worked on imported cars before and have the same level of ODIS software as VW UK, as well as VCDS (Ross Tech). The only difference between the UK and Jap Passat seems to be the MIB2 unit (No DAB), and obviously no DAB antenna.
For DAB that I can get via my phone on android auto, it isn't worth swapping the MIB2 unit and having a DAB antenna installed (but its possible)
I had an interim service and full inspection at ADS, they confirmed the car is in pristine condition (even underneath, due to the anti corrosion treatments the cars have in Japan) so I have peace of mind now that our vehicle can be supported at last!
I hope some of this information helps someone else researching about buying a VW from Japan!
Tom