Usually you can do geography A level if you haven't done it at GCSE, however, it may be worth getting in touch with the institution you wish to study at to check their requirements. As for extended writing, you'll most likely have a piece of controlled assessment to undertake (I do not know of a board where this is NOT included). The controlled assessment piece will be a few thousand words, but you have quite a bit of time in which to get it done. The exams also consist of shorter and longer (more essay) style questions. If you know which exam board you will be following you can view the specification for your course on line, and probably have a look at past papers to get an idea of what the essay style questions include. That'll also give you a good idea of what you topics you will cover.