I don't know how statements work post-16. But if the statement continues how can the school withdraw SN support as you say they have done? Surely you have a case against the LA?
In any event under the Equality Act (2010) the school has a duty to make 'reasonable adjustments' pupils to avoid them suffering 'substantial disadvantage'. As my DS didn't have a statement, that was the grounds (well, one of!) that we claimed disability discrimination against the school. (I think in your case the school would still have a duty to make reasonable adjustments, but as your DS has a statement, in deciding if the school was acting reasonably, the tribunal would have regard to what the LA are responsible for providing/funding.)
In my case we won on the grounds that DS's school had failed to make reasonable adjustments for him in Y12. Basically they didn't put in place support, and didn't monitor his (lack of) progress. Even worse, when they knew he was struggling - having meltdowns in lessons - (upset rather than disruptive), failing to complete classwork or homework, not managing to sit all of his mocks - they didn't tell us, pretended he was doing well and said he was on target for his predicted grades of AAAB. (He got CCDE). The school couldn't produce any contemporaneous records of support, and the SENCO at a CAMHS meeting even assured the doctor there that DS was fine and progressing well,when he'd just done disastrously in the mocks he did manage to complete. Luckily for our evidence, though of course the SENCO had no record of the CAMHS meeting, the doctor did...
But in our discrimination case we also claimed it was 'discrimination arising from disability' not to allow DS to study A2 Maths, but we didn't win on that one - the judge thought the School could make its own 'professional judgement'. We applied for leave to appeal to the upper tribunal but the judge, whilst giving the school a hard time at the hearing, didn't give permission, because he said that as DS was by then at uni, the costs involved in rehearing the case weren't 'in the public interest.'
We also claimed that the school refusing to let DS do a Y14 at the school was discriminatory, but at the first hearing we were told the tribunal had no jurisdiction over that claim because it was an 'admissions issue.'
So we actually won on what we thought was the hardest claim to prove, because we had to prove that the school was 'unreasonable'. Given the claims we didn't win on, I don't know if a tribunal would find it discrimination for your DS not to be allowed to study physics, or to study for a 3 year sixth form. A couple of the judges we came across in our protracted tribunal saga (about 20 months worth), to our mind were too ready to let the school 'get away' with the 'exercise of their professional judgement' argument, which we thought, and still think, somewhat negates the protection that the Equality Act is meant to give.
I don't get the funding problem with 3 AS subjects not 4 - I thought it was not that unsual to do 3 subjects. And I know that my nephew who has just gone into the sixth form (state comp) is definitely only doing 3 AS levels and no 'extras' which his school have agreed to, and I imagine they wouldn't have agreed if they couldn't get funding. I think bbkl may be right that full time ed is 12+ hours teaching time - though for my purposes it was whether DS qualified for a free bus pass, not if the school were getting funding!