1 - Switch to mathematics and physics only if you want to study 50% of a maths degree. They are quite different in style. It being well-regarded is irrelevant if you don't enjoy studying it.
2 - Yes you would be able to do a PhD if you got a decent grade (same as anywhere). Look up any university physics department's staff - they're definitely not all Oxford/Cambridge graduates.
3 - It's usually very easy to switch between MPhys and BSc after you start.