Just a few updates,
Using my method of watching the Ending Movie through Arcade Mode in between attempts of these championships,
It took me 5 attempts to win all 4 prize cars in the UK vs Japan Championship (the Red del Sol LM was the car that repeated) and 5 attempts to win all 4 prize cars in the UK vs US Championship (the Purple RX7 LM repeated...)
I'm convinced this is an effective streak breaker.
I now have acquired ALL of the prize cars in these championships! 
I don't plan on testing this method anymore, I've accomplished all of what I wanted to accomplish. I'm looking forward to seeing if anyone else finds this method useful.
Next mission for me is I want to unlock the Yellow NISMO 400R on my other game save. Considering there's 8 IA Licenses, I'll win 7 of them and continue to repeat the 8th until I get the car. I'll let you know how many attempts this takes!
Good luck everyone!
Edit: Yellow 400R on 3rd attempt
Last edited: Apr 28, 2020