GCSE science subjects compared to A-levels are completely different, don't base it off on how good you are and also definitely not just on a mock. A-level Physics usually require you to take maths alongside it due to the heavy maths content. The maths in GCSE physics currently is literally just easy and simple maths that you apply in a physics content, a-level physics go way beyond that.
Unfortunately, most colleges and sixth forms may not accept you onto the physics course if you dont score higher than a 6 in maths and even then they may persuade you to choose otherwise.
Usually, phsyics are stricter than maths when it comes to your maths GCSE grade because of how difficult it is, and you'll find many physics students take further maths too.
You could get away with not having a great maths GCSE with your other science subjects, but probably not Physics since they might require you to take A-level maths too.
That said, go to your sixth forms open days and attend as many taster sessions they offer, its how you find out what they require for GCSEs to enrol into the course, and they'll offer better and more specific advice.
Also consider what you want to do for university, there's no point taking such a difficult subject and ending up with a bad grade if you're not planning to explore further.