defo do a foundation, especially as you havent been able to do a full a level in art.
I did the same as you at GCSE and got A*A*, then did Applied Art&Design at A level getting AA, also an AS level in Fine Art getting a B. Ive just finished my foundation getting a pass with distinction so all is well BUT even with the good GCSE and A levels grade my portfolio wouldnt of been good enough to get to uni before the Foundation.
So basically go and do a foundation! its free if youre under 19 and you do it in one of your local colleges. You will have the opportunity to develop your drawing skills(which id say is pretty darn important) and learn all kidns of crazy new tchniques and skills that a levels dont offer. soooooooo go for it!
EDIT
Dont worry the foundation courses wont be botherd about you not having complete a levels in art. Put a variety of work in your portfolio. Observational drawings are a must, any life drawing would be a plus - dont panic if you havent done any, most people dont get to do it until Foundation level. Sketchbooks are always good to look at as well, that way the tutors are able to see how you have dveloped an idea. and if you have any 3d work, photograph it, dont take it to the interview.