There are two styles of address for academics: formal and social. In most circumstances the social form of address is used - that is, a polite but slightly less deferential style of approach than the very rigid form that was followed in the last century.
The formal styles of address may be found in the published edition of Debrett's Correct Form.
English universities have an old, established hierarchy, which differs from many of their foreign counterparts. At the lower end of the career path are lecturers (who often, but not inevitably, hold a PhD), senior lecturers and readers.
Professors are senior academics. They are said to hold a "chair" in a subject or subject area. Chairs may be established (i.e. not tied to the individual that holds it, and therefore passed on down the generations) or personal (i.e. linked to an individual).
Emeritus professors are professors who are officially retired, but still active within their universities.
Deans are heads of faculty (a collection of related academic departments).
The chief executive of the university is generally known as the Vice-Chancellor (sometimes the Principal or President, etc. see below).
The Chancellor of the university is normally a prominent public figure, who acts as a ceremonial figurehead (the Chancellor of Cambridge University is HRH The Duke of Edinburgh; the Chancellor of Oxford University is the Rt Hon Baron Patten of Barnes, the last Governor of Hong Kong).