Go for it. Sounds like you know what you want and why.
When I went to a sixth form college, a friend/acquaintance resat her GCSEs though she had enough to enter A levels. At first we were all like why, what's the point but she explained that her grades though were enough, limited the subjects she could take at A level and because she needed some of the subjects for her course at university, this influenced her decision. In the end, she ended up doing extremely well the second time round getting better grades, I'm talking about A*- B which definitely gave her more options at AS. In the end, a lot of us ended up repeating year 12 so we're all in the same year again lol instead, this time she had more options to what subjects she could study at AS.