When is the 卿 (Lord) honorific used? And does it differ between writing and speech?

Examples would include:

Lord Halifax, ハリファックス卿

Lord Kelvin ケルヴィン卿

Lord Thomson トムソン卿

etc…

I’ve noticed sometimes it isn’t appended, such as:

Lord Shang, which, for the title of his eponymous book, is rendered as 商君書

Is there a reason why?