What British English words and colloquialisms are not understood by American English speakers?

I have been working closely with both Americans and British for mre than20 years, and I can say that they understand eeach other very well and ith no real hardships

That's a hard question and no definite answer can be given. The presence of wordlwide media such as internet and TV reduce the probability of such greatly. What an American speaker might not understand could be understood by another. But generally, I'd say idioms which are used perhaps in rural areas.