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UPDATE - TVLicensing were OK when I explained I had thrown out the TV cos my son wouldn't do his homework, and I invited them to visit and check.
Chap turned up unannounced a couple of weeks ago, and seemed surprised when I welcomed him in. He saw there was no TV or cable box etc, and son was on iplayer on my laptop at the time. I could have shown him the browsing history to prove no live-tv viewing. He seemed quite satisfied and I have had no other visits, letters or calls.
If you refuse to let them in etc, they will be suspicious, I think if you genuinely don't watch TV they are fine. Or maybe you have to be noticeably eccentric too lol.