My advice is, "don't".
If you have a test tomorrow, then it's probably not a good idea to throw in extra information at the last moment. The chances of you getting the expressions wrong or confused are extremely high. These are things you can not paraphrase.
On top of that, throwing out some British slang in order to "impress" the examiners will have the opposite effect, especially if your standard English is of a medium level. If you're not relaxed and familiar enough with English and its nuances, then trying to handle idioms, figures of speech and slang would make you sound robotic or comical.
Sorry to rain on your parade, but this is how I've experienced it. Make passing the test with proper English your only priority. Good luck.