Your example sounds very reasonable to me, but do you think that this is ever taken too far?
Coming to the US as a student from Europe (not the UK), I noticed that some instructors (mostly TAs) took this effort to give a "positive" response to any answer in class to quite an extreme. I can think of many instances where a student's answer showed a very serious lack of understanding, and the instructor would say something like "okay, that would be one way to think about it, but how about... " without every correcting the student.
Two negative consequences I saw were 1) a lot of missed opportunities for correcting students' misunderstandings 2) students seemed overly sensitive to any kind criticism (I had a German friend who was a TA and had quite a bit of trouble with this)
Again, none of this is incompatible with your advice, I just wanted to take the opportunity to see if anyone has noticed this as a common problem.