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.

for ages: if you want to say that you haven't met your friends or someone for a long time, you could say: I haven't seen you for ages. under the weather: feel a bit ill John said to his teacher: Mr. green, I feel a bit under the weather today, can I have a day off to see doctor? over the moon: very happy Jack feels over the moon when Manchester United gets the champion of UEFA. couch potato: people who sitting on couches, watching tv everyday Lee is a couch potato watching TV all day long. I could only remember these expressions and If I remember more, I will add in.

'Stiff upper lip' was an expression used fifty years ago. It means 'keep calm, don't panic,' when you are in danger. 'Keep a stiff upper lip, old boy,' was common in military situations 'Keep cool' means the same. To be cool is different and means to be fashionable, modern. Good luck tomorrow. Don't learn this stuff, have a good sleep, then look up 'rhyming slang'.

Examiner!

"stiff upper-lippishness" I know it's not a real expression, but I read it on some blog and I pretty love it :D Guess it summarizes several cliches on the british quite well ^^