Is it a good idea to take up latin for gcse and A levels

Have to disagree with you there - Latin is very relevant and useful for somebody hoping to study Medicine. The vast majority of medicinal abbreviations and terminology is based on Latin, so I'd say it's well worth studying. (Although I may be slightly biased as a Latin A-Level student!)

No, you don't have to know the entire language in order to read a few words in the dictionary; but I would agree that knowing how latin words are constructed would help you understand those crazy long medical terms.