Championship doesn't work because it defaults all races to a practice/qualify/race format that relies on a pitlane to start from. Short ovals don't have pitlanes so the game crashes.
also, re: David Brown 908's no UI problem, I assume you're running ukdirt in nascar heat essentials rather than off a CD install. the essentials pack doesnt include a UI file, thats a mod-specific file but its usually big (20mb+, too big to post) so your best bet is to download a mod that includes its own UI.res (The Nascar heat essentials instructions on site like Texas speed racing, the Mod squad and the like recommend the iroc mod i think, thats self-contained and fairly small) - copy that UI into your ukdirtbanger folder and it should get you going.
cheers, hope it works for u ........ Wrighty