Not owning an actual TV set doesn't exempt you from buying a TV license. If you own a computer or mobile phone you need to buy a license for £145 per year. It's basically a flat tax imposed on every household.
You are correct in pointing out that a license is required no matter what device is used to watch or record broadcasts. However, just owning such a device does not mean that you have to automatically pay. If you use your TV only to watch DVDs, or your computers to only watch on-demand material, you don't have to have a license. The key word here is "broadcasts".
The tricky part is convincing the TV Licence goons that you either don't have any receiver, or that you don't use it. It used to be that you could have the RF unit specially mangled as proof you don't watch broadcasts, but now with set-top boxes and TV tuner cards and iPlayer and whatnot, it's a lot harder to demonstrate conclusively that you don't use any BBC services.
So they just keep sending you threatening letters.
