As a rule I've always dismissed JWs politely. I see no need to insult them or use clever put-downs (tempting though it is, sometimes!). The one exception was a number of years ago when there was a big story about two JW parents who refused to allow a life-saving blood transfusion for their young child. As far as I remember, other family members whisked the child off to (I think, but may be wrong) Cyprus to have the transfusion, so all ended well. A few days after this some JWs came to my door, and on that occasion I really did give them a hard time.
I haven't lived in the UK for ten years, and during that time I have had only two visits and a hand-written letter (which was rather odd as I don't know where they got my full name and address). In each case the JWs were evidently British expats rather than nationals of the country in which I live.