Keeping in mind I'm american and only lived in the UK for a few years (a few years ago)....
It's basically the same thing as Sweden where you're paying for a license to receive broadcasting. Many years ago (20+?)it was only BBC television (telly, as they're fond of saying), and it was considered a public service. There used to be no advertising, so the BBC needed something to pay for the programming. Think of it as mandatory donations to PBS, except the quality was much much better (subjective), and that it was the only thing on.
Nowadays, there's commercial broadcasting, I believe ITV and Channel 5 are commercial so it's less of an issue, but BBC (three stations I think) are still commercial free. Those are the only regular "over the air" stations I remember.
With cable and satelite (Is Sky the only option still?) it's even less of an issue, so the "TV Tax" is more resented, it seems.
Additionally, while I lived there, there was an issue of how the money was being spent. I can't remember the specifics, but it seemed there was a problem with how much of the budget was being "wasted" on enforcement.
Going from memory here and an American's viewpoint, so any Brits, feel free to correct me.
-Ryandor