Things may vary from subject to subject, but most people (particularly the 5*s) want to attract the best students irrespective of where they come from. A good PhD student will help push forward their supervisors research. When academics teach a high flying undergraduate they will be keen to recruit them for a PhD, but they'd be just as keen to recruit a high flyer from somewhere else. The Golden Hello scheme at Southampton appears to have been designed to help the university hang on to some of its very best students. I am not sure if this scheme is running this year.