There is no module system....its just that most schools do it that way to split up the work load and to make it manageable. Bristol GS do 8 in the first and 6 in the second. You can take all of them in one week if you like although its a bit stressful. The CAA require however that you pass them all in a certain number of sittings.
The exams are sat in the the first week of every month starting on the monday...there are usually 2-3 a day but the module system helps to break this up a little.
The brush up course definitely helped me for module 1....its really helping you to pass the exams...you should really know most of it before you get there.
I think its also the case that you have to do it through an approved school or the CAA wont allow you to sit the exams...so make sure you do it through oxford or something.
Good Luck
Tom