How much of my personal statement should consist of academics and extracurricular activities?

With regards to GCSE and A Level results I would say try to mention them as little as possible as admissions officers already have access to this information so wasting precious words on an already tight word count is not ideal! However, it's good to mention academic achievements in terms of things that have inspired you to apply for that particular course. This might be a particular part of an A Level subject syllabus, a academic project you've done, or a book/academic article that fascinated you. Whatever you do mention it's important to not spend much time describing it and focus on explaining what aspects of it influenced your decision to apply to this particular course.
The same principle applies to extracurricular activities. Focus on what skills you've learnt from them and how they are useful moving forwards into studying the degree you want to. For example, a medicine applicant might have experience shadowing a doctor. The admissions team are not that interested in a list of clinical procedures the student observed but are more keen to know why this experience made the student want to study medicine. The extracurricular activity does not have to be directly linked to the potential career, for instance if you have played a team sport for years the team work and interpersonal communication skills you've gained is applicable to many careers. So in terms of how much of your personal statement should be dedicated to academics and extracurricular activities it depends on how much you've gained from the respective things. I would say focus on what links to why you are applying for the course, and if I had to give a ratio I'd say go 60:40 extracurricular to academics but this really depends on your own application and what you have gained most from.