No matter if you master a specific variety of English (British, American or Australian). If you want to take a Cambridge exam (ESOL Examinations – Cambridge English Language Assessment), relax and…be consistent. Use the same variety and try not to mix them up, saying for example you’re not wearing pants but just shorts. It could be confusing and embarrassing, especially if you’re a girl.
The new FCE exam will be shorter and includes a compulsory essay (140-190 words) instead of a letter/email in the Writing part 1. Here you find a nice webinar to get ready.