Within your field, it's probably more important to emphasise who your supervisor(s) were during your PhD. At least in the UK, the prestige of the university is less important at that level than the prestige of the other academics you worked with (and, of course, the quality of your research). In any case, it would be odd to see it in an email signature. I'd read it as (slightly desperate) bragging.
One related point is that people who have undergraduate degrees from Oxford or Cambridge may receive an MA rather than a BA under certain circumstances. These people might list their first degree as "MA (Oxon.)" or "MA (Cantab.)", listing the university to distinguish it from a "real" Masters degree (and possibly to do a little bragging of their own).