Even at Oxford I only ever heard it used humorously.

Only time I've seen "Dean" used was for the head of a faculty within a university (a collection of departments in the same field e.g. engineering).

In my Cambridge college there is one 'dean' (not the same as the Master or Mistress of the college - who is essentially the figure-head of the college) - who is responsible for the students welfare. Any member of staff formally associated with the college is a 'fellow'. Each speciality have 'tutors' and 'senior tutors' as well as one 'director of studies'.

Wikipedia says that a 'don' would apply to a 'fellow' or 'tutor'.