Typical theoretical physics modules you'd study (going on my experience at Manchester) would be:
- Random Processes in Physics (basically probability applied to physical systems)
- Advanced Dynamics (further topics in Newtonian mechanics and Special Relativity beyond core physics)
- Lagrangian/Hamiltonian Mechanics (a different approach to classical mechanics without ever writing F = ma ever again - you'd probably also study a branch of maths called the calculus of variations in this course, as it underlies Lagrangian mechanics)
- Some sort of mathematical introduction to quantum mechanics - so learning about linear algebra and vector spaces as applied to quantum mechanics, you'd probably get an introduction to Dirac notation here)
- More advanced courses in quantum mechanics building on the previous one.
- Nonlinear dynamics and chaos
- Electrodynamics (an advanced Electromagnetism course, where you'd probably also get some exposure to the formal presentation of special relativity using things called tensors so you'd look at Electromagnetism and Relativity, and it'd also help to prepare for the type of maths you'd need in General Relativity)
- General Relativity
- Quantum Field Theory
You'd also do a couple of advanced mathematical methods courses which cover mathematical techniques in theoretical physics and you may also do some additional programming work (everyone will be taught to program - but at Manchester, 2nd year Theorists replace half of their lab content with a computing project which will involve some programming and mathematical modelling)
The main difference between Maths/Phys and Theoretical Phys is that in Maths Phys you may not get the opportunity to study (m)any of the theoretical courses I've described above. Of course, if the University you end up going to has theorists in their maths department, you may be able to study some then. What options you'll be offered exactly will depend on where you go and the things that the department does research in, particularly in the later years. Course structures will vary between universities so it's hard to generalise too much.
Manchester puts their student handbook on the department website I've linked to the page with syllabuses here. The courses marked (M) are the Theoretical options.
http://www.physics.manchester.ac.uk/undergraduate/courses/course-handbook/10_syllabuses/If you also click this link you can look at the structure of the degree course, semester-by-semester so you can compare how Maths/Phys and Phys/Theory differ (at Manchester, at least...)
http://www.physics.manchester.ac.uk/undergraduate/courses/course-handbook/7_programme_structure/Hope this helps. Theoretical Physics is a great subject, it's also very very hard
Also, Manchester ftw