The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 6, 'Capitals'
6.2 Titles and Dignities
Capitals are used for titles and dignities when these appear in full or immediately preceding a personal name, or when they are used specifically, but not otherwise:
The Archbishop of Canterbury and several other bishops were present, but Bishop Wilberforce was not.
When, after a first full reference, or with such reference understood, a title is used incompletely but still with specific application to an individual, the capital is retained:
The Archbishop spoke first.
A word or phrase used as a substitute for, or an extension of, a personal name also takes initial capitals:
the Iron Duke, Alfred the Great, the Dark Lady of the Sonnets