As long as you are consistent in your pronunciation it doesn't matter if you use US English or British English.

Thank you, Paul, I've got you!

You don't have to use British pronunciation to do well on that test. American pronunciation is just as valid, honestly it would be ridiculous and churlish in the extreme for an examiner to mark something as wrong because a person used the American spelling/pronunciation/word, given there are more than 300 million people who speak that way. If you've been using the American pronunciation, stick with it. Changing at the last minute to something that doesn't feel intuitive is more likely to cause you problems.

She is Russian, a teacher of Cambridge courses, she prepares pupils of the7th grade for Cambridge English test (KET).

Is she a native speaker trained specifically for this test prep?