After a quick Google search I came across this topic and wow, what terrible advice! I had to sign up and post. I don't mean to insult who has responded to this but it is obvious that you have little to no experience in this field.
First, for a CG course a BTEC is more then adequate and not a bad thing to have compared to A levels.
This is because a university's department which specializes in CG and art will like to see that you have done a more concentrated course in that field. A BTEC in some form of digital media or art is better then German, Philosophy and History A levels in there eyes.
A big thing that CGI courses look for is that you can draw so if you must do A levels, choose Art as one of your choices.
The CGI industry is a competitive place as with most art related jobs. You cant leave university with a 2:1 and expect to get a job strait away. You have to be good! At least 100 people graduated from my university with a CGI degree and I can tell you 90% of them were far from getting a CG job unless they worked really hard and got better.
Because of your choice in education, your art skills will not increase quickly. Once at university you will see a big disadvantage.
Of course if you are not 100% sure about doing a CG course then your choices are good. You can decide to jump to another course. But if your going to be paying around 20k for a degree in CG, then you need some skills to build you up to it.
In regard of develop you portfolio in your spare time then sure. But you still wont have experienced teachers giving you advice on your art.
If you do some extra night lectures then sure but whats the point. If you are committed to enter that field then just do the BTEC.
Right I think thats my rant over. I hope this will help you and all who see this out.
A little about the rout I took.
I took an additional foundation GNVQ in art and design
BTEC in 3d design
Took a BSC course in computer animation and Special FX
Im a recent graduate and work currently as a computer animator.
My brother in law is wanting to work in a CG field and he is about to start he BTEC.