Once you reach a certain level of skill and experience, like Schneier has, you sort of outgrow titles. I'd say he's a researcher and an educator.
Have you ever read his articles? I think that's as apt a title as any for Mr. Schneier.
Without doing any research on what he actually does at BT, that's probably between senior consultant/advisor and security evangelist, which he would speak about his cryptanalysis skill and be the go-to person when someone needs advise on security matter.