They don't go to prison for not having a TV licence. They are fined (I believe incarceration is an option but the courts apply a modicum of common sense in this regard).
However, some people still don't pay and ultimately end up in prison for non-payment of the fine.
There has to be a sanction for non-payment of fines though. I think the proposal to change the status from criminal to civil is very sensible however. The worst that can happen then is you have a judgement against your name for 6 years.
You don't need a license to own a TV that's right, but damn if you accidentally tune it to ITV4 for 5 seconds while hooking up the DVD player, or if you click on a BBC News (or Sky news for that matter) live stream on the web in your unlicensed house, you are a criminal right there. Literalty that is a criminal offence. Is that proportionate?
However, some people still don't pay and ultimately end up in prison for non-payment of the fine.