Personal titles

Royalty

Use Roman numerals with names. Only the British monarch retains the initial cap in all circumstances. Other monarchs are capitalised only when the name is used.

In general, prince and princess have an initial cap if used with the name. At first reference, Prince William should be described as the Prince of Wales.

Capping up

The Royal Family, but royals, royalty, royal baby, royal wedding

His Majesty, the King

the palace, but Buckingham Palace

Last Names

Meghan is no longer Meghan Markle. She is Meghan, the Duchess of Sussex.
Similarly, we no longer use Kate Middleton. She is Catherine, the Princess of Wales ('Kate' only in headlines).

Names at first and subsequence reference

*Kate can now be used in headlines. Never in the copy.
** In Scotland, the Prince and Princess of Wales are now also known as the Duke and Duchess of Rothesay

Note: Prince Andrew is no longer referred to as His Royal Highness (link).
Prince Harry and the Duchess of Sussex aren't referred to as HRH either (link).

Possessives

The possessive of Charles is Charles’s
The possessive of duchess is duchess’s
The possessive of Wales is Wales’
The possessive of princess is princess’s

Titles (people)

Lord: The forename is not included when naming a marquess, an earl, a viscount or a baron. An exception is when two people have the same title. So Lord Alan Smith and Lord Peter Smith may be used to distinguish.

Lady: The title 'Lady' is conferred on the wife of a marquess, earl, viscount, baron, baronet or knight, or the daughter of a duke or marquess or earl. In the first of these groups, the first name is not used, so it is Lady Hermon rather than Lady Sylvia Hermon. But those in the second group do include their first name in the title (eg: Lady Antonia Fraser is the daughter of the late Lord Longford, the surname having changed when she married). A baroness may use that title eg: Baroness Simpson, but can also be referred to less formally as eg: Lady Simpson. We would generally use Baroness at first reference, then Lady. 

Sir/Lady: When referring in text for a second time to a knight of the realm (eg Sir Keir Starmer) use the accepted nomenclature of Sir Keir. 

Honorifics

Use Mr, Mrs etc except for convicted criminals and also journalists, sports people, authors, actors, artists, musicians and entertainers in their professional capacity.

Court reports in the UK and abroad, should give defendants an honorific unless and until they are convicted.

Miss, Mrs, Ms: In choosing between Miss, Mrs and Ms, try to find out what the person themselves use, and stick to that. Avoid foreign honorifics (eg: Herr, Madame or Signora). Default for Ms if preference is not known.

Mx.: An increased number of organisations are adopting the gender-neutral honorific Mx for people who either idenify as non-binary, or if they don't want to be identified by gender. A descriptor for the title might be required, while audiences get familiarised with the term.
Also see: Gender and sexuality

Dead people: There is no ban on using honorifics with the dead. It's a matter of judging what is appropriate eg: A man murdered in front of his family does not immediately become 'Smith'; he remains Mr Smith. It would be difficult to defend a court report where the victim was 'Smith' and the alleged killer 'Mr Jones'.

Abbreviate titles when used as before a full name

But use the full title when referring to an official for the first time