I preferred sixth form far more to GCSE, trust me GCSEs really are the worst part of school life because you're forced to do **** you really don't wanna do, as well as doing way more exams. At GCSE I did 22 exams and for AS it was 7- and if all goes to plan then I'll only have 4 next year. Of course AS is way harder than GCSE, but the passion for the subject should motivate you enough.
You're also given a lot more freedom from teachers and frees, which multiplies in U6. They take you more seriously and classes are way more fun and you get to know the teachers so much better.
Try out Sixth Form, you'd regret it if you didn't - and if you don't like it then it's only 2 years!