Hi Johnnie,
You have two real options for getting a PhD in the UK;
1. Apply for an already funded project
2. Make your own project and apply for funding
If you choose option 1, places like findaphd.com is your best bet but they don't have an astronomy sectrion (they do have astrophysics). This method usually has funding and a supervisor already in place. Though you don't get to choose the project and it some supervisors can be awful.
Option 2 is far harder. You need to find your own project, a willing supervisor and then apply for funding. This is far harder but has more academic freedom. Don't much about this way but you can ask again iof you go down this path.
On a subject basis, I think you can easily apply mathematics to astronomy. (note I am an engineer with no experience in astronomy or maths). From my experience astrophysics is very mathematics heavy so you will have some of the right skills already and that is how you can sell yourself. Research is becoming ever more cross-disciplinary, so moving fields is possible if you have the right skills or knowledge.
You ask what field you should be targeting? - preferably a field that you think you can dedicate three years of your life to. Not joking you. The best topic will be something that you are motivated about because three years is a loong time.
Hope that helps and good luck,
rewt