You should also note that the problem with these overly specialised A-levels is that they don't really open up your options much, unlike more traditional subjects such as the sciences. Also you have to keep in mind that with these you'd have to go from doing 10 general GCSEs to 3/4 very specific A levels, I dont think many people are that sure of what they want to do straight after GCSEs.
Not even mentioning problems with existing specialized A levels (I've talked before about these, things such as Marine Biology..), such as lack of schools / resources, universities maybe not recognising them.
About theoretical physics as I'm personally very interested in physics, how would the math content be done? There's lot's of advanced math that im not sure belongs in an A level, also how much of it would overlap with normal physics? Something I'd personally like to see is "Further" subjects that arent math, eg. Further Computing, as I really like computing but I feel like the current A level is a bit of a joke when it comes to actual computer science.