I have no idea how far you'd get with that argument *if* TV licensing pursued you after you informed them you didn't need a licence. At the end of the day, they have to prove that you need a licence - I don't know what the courts would consider sufficient "proof" in that regard.
And this is the stickler really. You can be guaranteed TV Licensing would say you need a license. Especially the people that knock on your door!
I don't have my TV tuned into anything so if they knock on the door I invite them in (This has happened once as I am never home when they come knocking) to have a look. I wrote to them once to tell them I did not need a license and they wrote back saying someone would be round to confirm this. They never did (or I wasn't home when they did and they didn't leave any notification of such) and the letters never stop so these days I just ignore them and carry on with my life.