When I did chemistry at A-level (2 years ago) the two sixth forms I applied to both required a good chemistry GCSE to take the A-level. Some sixthforms will let you retake Chemistry GCSE alongside two other A-levels, then you can do Chemistry A-level after that.
Just as a warning though, chemistry A-level is a very difficult subject, and I struggled with it, even though it was my best GCSE subject and my dad worked in a chemistry lab for 24 years of his life, and had been basically training me my entire life, so if you are struggling with it at GCSE, you might have difficulty with it at A-level. But if you think you can do it, then go ahead - it is much more interesting than the GCSE.