Re: I think a paid service would only work
I suspect it will be based upon detection rather than enforcement.
Access logs will tell them the IP addresses of everyone using iPlayer, which they can map to an ISP and use geoip to get approximate location.
The BBC already send threatening letters to anyone who doesn't have a license, on the assumption everyone watches TV. They could extend this to send out letters "reminding" anyone without a license within that approximate location.
Maybe demand the ISP tell them which household had been assigned that IP address and take a few people to court to make examples of them.