Someone's been telling you porkies! Just enter 'phd mathematics' into Google (restricting your search to UK sites) and you'll find dozens of universities offering PhD opportunities in mathematics. (I'm sure the same will apply for economics). Perhaps the confusion comes about because some universities prefer to use the more appropriate title of Doctor of Science (DSc), rather than that of Doctor of Philosophy (PhD), as the title awarded for original research within the fields of science?
[ Posted by someone who had to write a university thesis on 'Diophantine Equations' ! ]
Chris