I think it will be okay if you were to go ahead with A Level maths, there’s many topics that are dropped from GCSEs and new ones added in A Levels, so you’re good to start in A Levels. (Since you aren’t starting from 0 knowledge in maths).
There is a book available which is designed for GCSE students who want to prepare for their A-Level Maths, and that only covers the GCSE content that's relevant to A-Level Maths:
You may also wish to have a look at the FSMQ Advanced in Additional Mathematics. This is a level 3 qualification sat alongside GCSEs which is an excellent bridge for A-Level, and introduces some of the more advanced ideas in a simpler way.