I went over the past TMAs and did the specimen exam paper for Planetary Science today. I then marked myself to see how well badly I'd done. I ended up with about 30/100, and that's being generous to myself. Most of the questions I couldn't answer or thought I could, but when I checked it turned out that I had screwed the whole thing up.
So, I'm not taking the exam.
I shall now be moving on to studying Maths for Science and hopefully improving myself that way instead.