Quote From TreeofLife:
Where are you applying to? Most UK PhDs in the area will be advertised ones, like on findaphd.com. It's unusual to write your own proposal unless you have your own funding.
Just to add, this may be true of STEM subjects, but in the arts, humanities and social sciences, there are very few advertised projects. Most of the funded places in these fields come from AHRC and ESRC funded DTPs, and the research proposal is a very important part of the application process.