Instead of typing Keir Thomas when composing a new mail in the Mail app, in order to have the address autocomplete via an entry in your address book, try typing just k t (note the space between the initials; kt won’t work). You can even type the initials backwards: t k.
If you’re trying to email Katherine Smith, try typing kath smith or k smith or kath s.
Get the idea? If there’s more than one suggestion, use the Up/Down cursor keys if necessary, and hit Return to select the one you want.
This seems to work in the iPhone/iPad Mail apps too.