Thinking about taking media studies at A level- good idea/bad idea?

I don't think it's a good idea. Both the top universities, and many employers, look on qualifications in things like 'media studies' as worthless. You want to work in the media, but perhaps you should think about where exactly you'd like to work. If you want to be a journalist, perhaps take English, and another language (if you're good at French, take that - not many people take languages now, but there is a huge demand for bi-lingual people out there). A specialist topic like geography, history, or a science would also be good, and give you something to distinguish you from the thousands of other applicants when it comes to applying for a job in the media.