2. As I said, I didn't prepare at all for the compass test. But, I dont't think that there is somebody who bought all of the preparation programme, and can say that one of them is the best. The idea is to prepare, the actual compass test I think is registered as a mark, and can't be used by somebody else. The main ideea is that they all do the same thing, but slightly different. My best advice would be to look at the demos, an choose which one looks more ok to you.
3. The English grammar test isn't so bad, but you need to have a minimum of knowledge and understanding of English language.
About your first question, the BAA site doesn't mention any minimum grades, but they say that they are testing the knowledge of math and physics. For example, in my high school I had a minimum training in physics, but managed to acquire them from my other "hobbies". So my grades that can be proven with papers are low to none, but practical I can demonstrate I have the knowledge and understanding of what i know.