I'd say these three are just fine. If Cambridge and Imperial don't have a problem with these, then really, you should be fine. Generally, you'd need a fourth subject at AS, though. Check with the universities you're interested in.
If you'd really *want* to study any other subject, then do it. If you're applying to the LSE (in case you somehow decide you'd rather do say, Stats w/Finance or something!) or Cambridge, check with them. For Cambridge, specific colleges have their own requirements. For instance, Business is not a very well regarded subject. Sciences, Maths, Languages and Economics/History, *are*, on the other hand.
I don't care what your GCSE grades are. There's a bit of a leap from GCSE Maths to A-Level Maths. Since you do seem interested, I'd suggest you start working through your "assigned" Mathematics textbooks asap. Use KhanAcademy and the forums here when you hit a wall, which you will, eventually.
While I haven't started university study yet, it is my understanding that "theory", "computation" and "experiment" are merely techniques for doing Physics. Further to that, doing a straight-up Physics course with extra courses in Maths and Computer Science should prepare you well enough for getting into these kinds of PhD programs eventually, if you'd still want to. (correct me if I'm wrong...)
The Theoretical/Mathematical Physics variants of these degree courses are, as far as I know (and I have read around a lot), essentially the same as their "Physics BSc/MPhys" counterparts, except that in the former, one does some additional mathematics modules instead of certain physics labs.