I’ve been dealing with patients for coming up to fifteen years, and part of the art of communication is judging how they would like to be addressed, and adjust my linguistics appropriately. There are plenty of times when a respectful use of Mr/Mrs ‘surname’ is entirely appropriate, but also times when m’dear, owd’un, chap, sweetheart, shug (when in Stoke) etc have all been terms that have been useful in putting a patient at ease in what can often be quite stressful situations. I do have a vague recollection of some sort of instruction about not using colloquial terms to address patients, but I like most of my colleagues would pretty much rather use our common sense and judgment than be told how to talk to people by someone who obviously has far more time on their hands than they probably should have.
In summary; it’s all about context, innit.