What is a "CFI"? (I mean in the UK context, not FAA)
I was once told (by a CFI) that any instructor who is authorised to instruct on any plane which the school happens to train in (which doesn't take much doing) can call himself a "CFI".
A&C: good points about instructors. Unfortunately, a PPL student has zero chance of being able to spot which type is which.
I would classify the instructor population a little more, however. If a student is reasonably technically minded and actually understands the aircraft and flight mechanics, there are many instructors who will get really p1ssed off by such a student. Others are happy to take such a person on board and enjoy the challenge.
I make no claims for being any good at flying, but I used to ask a lot of reasonable questions (mainly on why are certain procedures the way they are) and got treated very badly for it. Walking out of one school is a great way to blow away £2000 or so and I suspect many students never scrape up the courage to do that.