I've been a bit disappointed in them to be honest (they could, I suppose, be hiding in the fir trees out front with cameras trying to catch evidence of illicit broadcast TV watching), we get periodic letters that announce on the the back they're from TV Licensing which saves me the effort of opening them. I've no wish to confirm that I don't watch, I've cancelled the licence, which is a clear and unambiguous statement. Telling them some more won't change it and if I were lying, it wouldn't change that either.
I presume at some point we might get a visit. To be honest, though I disagree, I'd probably let them look on the grounds that being arsey to people who are just doing their job (and probably not loving it) is just being arsey. Though I always imagine that they turn up on your doorstep dressed in a Gestapo uniform like minor royalty at a fancy dress party. Probably with a comedy German accent. 'Ve hav detected ze tee-wee!' he'll shout as he parks the TV Detector Panzer on the driveway.