From my experience with that stuff, the classes are pretty much open and you can take on extra exams. So I would go for the version that is a little easier and if you're fine with that, you should be able to add on. If your school doesn't want to, I don't think you need a school to take the exams. You just have to work even harder on your own or pay another teacher to help you.
=290Pika is right. Why did you get a D? Nobody here can tell you, so answer yourself honestly: were you lazy, unable to understand the material or a combination of both?
If it's just laziness, there's a chance you can change it, but a lifetime of slacking is hard to reverse and A Level Maths is incredibly hard and demanding. People who get As at GCSE often don't pass the A Level because of the huge jump in difficulty.
How come you got a D? Is it work ethic, or just not understanding the material? Usually math classes build upon the previous ones (though I'm American, so it might vary from my experience). If you don't understand the material enough to pass the class, taking a more advanced (extended?) version of the next class might not be a good idea.
Now, if it was work ethic over understanding the material, then you might be fine so long as you keep up your work.