Fourth a-level as a private candidate.

My sixth form can't offer me Physics as it is full - and I NEED physics for the courses I want to attend/my future career. I have no other sixth form choices and this is the easiest to attend with my parents' busy work schedules and as it is walking distance.
No matter what my sixth form cannot offer it, and I've decided I'll sit it as a private candidate and just revise it fully after school and during my frees.
I wouldn't mind the spare subject I chose to suffer a little as it was not the a-level I originally intended on taking.
Would it be possible to achieve high grades in Physics? I'll have teacher support along the way. I'll obviously have to work hard and I will as I genuinely want to do this subject.
Honestly I just need some reassurance that if I put the work in I'll be good to do Physics as a private candidate despite going sixth-form anyways.

You have 2 equally good options:
1. Take physics as a private candidate as you mentioned
2. If it exists, a foundation year degree of the course you want to do that requires physics.
Im assuming, you going to prioritise physics over the other A Level that you have to take in its place. Since you dont want to do that A Level, can you talk to your sixth form to not do that subject, so you have more frees for A Level Physics. The reason Im mentioning this is because doing 4 A Levels are difficult, and people who start year 12 with 4, enter year 13 with 3. Unless, it is your school policy for all students to do 3 A Levels (and you cannot do 2 and sit one as a private candidate).

you will be good at physics as a private candidate.

My sixth form can't offer me Physics as it is full - and I NEED physics for the courses I want to attend/my future career. I have no other sixth form choices and this is the easiest to attend with my parents' busy work schedules and as it is walking distance.
No matter what my sixth form cannot offer it, and I've decided I'll sit it as a private candidate and just revise it fully after school and during my frees.
I wouldn't mind the spare subject I chose to suffer a little as it was not the a-level I originally intended on taking.
Would it be possible to achieve high grades in Physics? I'll have teacher support along the way. I'll obviously have to work hard and I will as I genuinely want to do this subject.
Honestly I just need some reassurance that if I put the work in I'll be good to do Physics as a private candidate despite going sixth-form anyways.