Im sure that's fine, ik at my school someone didnt do art GCSE the art teacher asked to see their personal artbook or sketchpad and they were fine with it and allowed to do Alevel.
Idk if youve got a place and can talk to your art department at your 6th form, but if i were you the first thing I would do would be to email the sixth form abt it
gl, dont rush the decision you have ages to decide and you can still change once school has actually started. Have you thought abou taking both and then dropping one?