Unfortunately UK dealers will almost always try and put people off a JDM car.
They are afterall, in business to sell you their product, and have a vested interest in making sure people wish to purchase a UK car.
I had exactly the same issue when shopping for three company Golfs. The VW dealers tried spinning me all sorts of crap to put me off imports. The list included - lights the wrong way round, engine air pipes different, guages different, trip computers different, and dealers would refuse to service it and stamp the book.
As it happens, we ended up buying two import golfs. Both came with higher spec than UK cars (heated seats, lamp washers, heated washer jets, climate control) - and both have been rock solid for four years. And both were cheaper.
One has needed a single brake light switch, which was sourced without issue from a main dealer.
And they have both been serviced without any question by VW, and the service book stamped.
It is often the case that members of car enthusiast forums, especially those dedicated to one particular marque, will often know far more about the car than any dealership.
This has certainly been my experience for the Golf.
As long as you research your intended purchase fully - which you are doing by asking questions of existing JDM owners, then you will be fully aware of all the differences.