Mathematics undergrad to theoretical physics phd
Yes (you might not even need the Physics GRE). Example:


  The usual minimum entry requirement is a first-class honours degree, awarded after a four-year course in physics, mathematics or engineering, or a three-year degree together with a one-year postgraduate course on advanced mathematics and theoretical physics. Part III (MMath/MASt) of the Mathematical Tripos provides such a course. Note, however, that entry is competitive and a higher level of preparation may be required for research in some subject areas.