Which Universities admit CS graduates for their theoretical physics course?
I can't speak for the UK, which has a different educational system altogether, but in the US, most programs would find a CS graduate "acceptable" for a physics doctorate. This assumes a good record overall, of course, and probably a few physics courses in which you did well. In the US, a CS student probably has a few such courses, though not advanced ones. 

But to actually be admitted you have to be better than "acceptable". 

In the US, the graduate program has quite a few courses available, primarily to prepare the student for comprehensive exams. But the normal path would be into a doctoral program, perhaps with an MS along the way. 

I would guess that in the UK the university would expect an applicant to be further along in the major subject, but others would need to verify.