Can a maths undergraduate apply for a physics PhD?
It's hard to give a general rule as policies will vary between institutions and countries, but this would probably depend on the specific Physics related content of your course.

From my own experience (UK universities for undergraduate masters Physics and Astronomy and then PhD Astrophysics), it is not common but definitely acceptable to get a place on a PhD course straight from a Bachelor's. Exceptional candidates (eg. having multiple publications before graduation) would definitely be considered.

In terms of switching subjects, you would need to show that you have the background knowledge and appropriate skills (numerical, computational, logical problem solving) for the PhD you are applying to, but if your Maths degree covers this then it should not be a problem. I imagine aiming for theoretical physics or highly mathematical fields such as cosmology would be quite easy to justify from a Maths course that included mathematical physics. A practical or astronomy related PhD may be harder to justify as you probably wouldn't have the broad background knowledge or lab skills. For computational projects such as gravitational waves or theoretical astronomy, you would need to have computing courses in your undergraduate degree, to an acceptable level, though the language you learned wouldn't matter too much as you can easily pick up a different language.