Hello /wsr/, semi-long post ahead, I don't know what to do with my life after I graduate in theoretical physics next year. Most of the physical theories I've encountered during my studies have disappointed me but I've found true joy in one thing... pure mathematics and I took pretty much any class on it that I could. While I should start searching for a PhD, I have no idea where to look because
a) I'm still not sure what area of physics interests me the most (more on this below)
b) Not one professor can help me; they all said to ask ex-students and search online

Relativity and Cosmology remain the only two branches of Physics that (I think) I still have a genuine interest in. The problem is that I only took one course on Special Relativity and one on General Relativity and barely scratched the surface of Cosmology so who knows what waits for me there. I'm also okay with taking an additional one year of Graduate studies and, after some online digging, I found the Perimeter Institute program which I'm at the moment considering but I really can't say it's something I strive for. Then I found the MMath/Mast program at the University of Cambridge and it's everything I ever dreamed of... but I can't afford it. Do any of you know of a similar course in Mathematics that a Physicist could apply to? Maybe something similar to what Perimeter has but for math? Is it possible to pursue a PhD in Theoretical Mathematics with a background similar to mine?